Add escalations table for issues

What does this MR do?

Related issue: #330277 (closed)

Adds a table for representing when issues should be escalated according to a project's escalation policy.

The structure of this table is meant to follow the one introduced for alerts in !64274 (merged) & modified in !65635 (merged).

Up migrations

% bin/rails db:migrate
== 20210730194555 CreateIncidentManagementPendingIssueEscalations: migrating ==
-- execute("CREATE TABLE incident_management_pending_issue_escalations (\n  id bigint NOT NULL,\n  rule_id bigint  NOT NULL,\n  issue_id bigint NOT NULL,\n  process_at timestamp with time zone NOT NULL,\n  created_at timestamp with time zone NOT NULL,\n  updated_at timestamp with time zone NOT NULL,\n  PRIMARY KEY (id, process_at)\n) PARTITION BY RANGE (process_at);\n\nCREATE INDEX index_incident_management_pending_issue_escalations_on_issue_id\n  ON incident_management_pending_issue_escalations USING btree (issue_id);\n\nCREATE INDEX index_incident_management_pending_issue_escalations_on_rule_id\n  ON incident_management_pending_issue_escalations USING btree (rule_id);\n")
   -> 0.0043s
== 20210730194555 CreateIncidentManagementPendingIssueEscalations: migrated (0.0269s) 

== 20210820171834 AddForeignKeysForPendingIssueEscalations: migrating =========
-- foreign_keys(:incident_management_pending_issue_escalations)
   -> 0.0048s
-- add_foreign_key(:incident_management_pending_issue_escalations, :incident_management_escalation_rules, {:column=>:rule_id, :on_delete=>:cascade, :name=>"fk_rails_0470889ee5", :validate=>true})
   -> 0.0106s
-- foreign_keys(:incident_management_pending_issue_escalations)
   -> 0.0022s
-- add_foreign_key(:incident_management_pending_issue_escalations, :issues, {:column=>:issue_id, :on_delete=>:cascade, :name=>"fk_rails_636678b3bd", :validate=>true})
   -> 0.0034s
== 20210820171834 AddForeignKeysForPendingIssueEscalations: migrated (0.0729s) 

% gdk psql
\psql (12.7)
Type "help" for help.

gitlabhq_development=# \d incident_management_pending_issue_escalations
                                 Partitioned table "public.incident_management_pending_issue_escalations"
   Column   |           Type           | Collation | Nullable |                                  Default                                  
------------+--------------------------+-----------+----------+---------------------------------------------------------------------------
 id         | bigint                   |           | not null | nextval('incident_management_pending_issue_escalations_id_seq'::regclass)
 rule_id    | bigint                   |           | not null | 
 issue_id   | bigint                   |           | not null | 
 process_at | timestamp with time zone |           | not null | 
 created_at | timestamp with time zone |           | not null | 
 updated_at | timestamp with time zone |           | not null | 
Partition key: RANGE (process_at)
Indexes:
    "incident_management_pending_issue_escalations_pkey" PRIMARY KEY, btree (id, process_at)
    "index_incident_management_pending_issue_escalations_on_issue_id" btree (issue_id)
    "index_incident_management_pending_issue_escalations_on_rule_id" btree (rule_id)
Foreign-key constraints:
    "fk_rails_0470889ee5" FOREIGN KEY (rule_id) REFERENCES incident_management_escalation_rules(id) ON DELETE CASCADE
    "fk_rails_636678b3bd" FOREIGN KEY (issue_id) REFERENCES issues(id) ON DELETE CASCADE
Number of partitions: 8 (Use \d+ to list them.)

Down migrations

% bin/rails db:migrate:down VERSION=20210820171834
gdk psql
== 20210820171834 AddForeignKeysForPendingIssueEscalations: reverting =========
-- foreign_keys(:incident_management_pending_issue_escalations)
   -> 0.0047s
-- remove_foreign_key(:incident_management_pending_issue_escalations, :incident_management_escalation_rules, {:column=>:rule_id})
   -> 0.0105s
-- foreign_keys(:incident_management_pending_issue_escalations)
   -> 0.0024s
-- remove_foreign_key(:incident_management_pending_issue_escalations, :issues, {:column=>:issue_id})
   -> 0.0043s
== 20210820171834 AddForeignKeysForPendingIssueEscalations: reverted (0.0221s) 

% gdk psql
psql (12.7)
Type "help" for help.

gitlabhq_development=# \d incident_management_pending_issue_escalations
                                 Partitioned table "public.incident_management_pending_issue_escalations"
   Column   |           Type           | Collation | Nullable |                                  Default                                  
------------+--------------------------+-----------+----------+---------------------------------------------------------------------------
 id         | bigint                   |           | not null | nextval('incident_management_pending_issue_escalations_id_seq'::regclass)
 rule_id    | bigint                   |           | not null | 
 issue_id   | bigint                   |           | not null | 
 process_at | timestamp with time zone |           | not null | 
 created_at | timestamp with time zone |           | not null | 
 updated_at | timestamp with time zone |           | not null | 
Partition key: RANGE (process_at)
Indexes:
    "incident_management_pending_issue_escalations_pkey" PRIMARY KEY, btree (id, process_at)
    "index_incident_management_pending_issue_escalations_on_issue_id" btree (issue_id)
    "index_incident_management_pending_issue_escalations_on_rule_id" btree (rule_id)
Number of partitions: 8 (Use \d+ to list them.)

% bin/rails db:migrate:down VERSION=20210730194555
== 20210730194555 CreateIncidentManagementPendingIssueEscalations: reverting ==
-- drop_table(:incident_management_pending_issue_escalations)
   -> 0.0062s
== 20210730194555 CreateIncidentManagementPendingIssueEscalations: reverted (0.0295s) 

% gdk psql
psql (12.7)
Type "help" for help.

gitlabhq_development=# \d incident_management_pending_issue_escalations
Did not find any relation named "incident_management_pending_issue_escalations".

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Does this MR meet the acceptance criteria?

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Related to #330277 (closed)