Add issue_id to epics

What does this MR do and why?

With the migration of epics to work items we will move epics to the issues table. To still keep a reference once an epic is moved, we have the issue_id now as part of the epics table.

Issue: #434528 (closed)

Migrate

main: == [advisory_lock_connection] object_id: 183000, pg_backend_pid: 83761
main: == 20231214095546 AddIssueIdToEpics: migrating ================================
main: -- add_column(:epics, :issue_id, :int, {:if_not_exists=>true})
main:    -> 0.1288s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("ALTER TABLE epics ADD CONSTRAINT fk_893ee302e5 FOREIGN KEY (issue_id) REFERENCES issues (id) ON DELETE CASCADE NOT VALID;")
main:    -> 0.0029s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0001s
main: -- execute("ALTER TABLE epics VALIDATE CONSTRAINT fk_893ee302e5;")
main:    -> 0.0107s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0005s
main: -- index_exists?(:epics, :issue_id, {:unique=>true, :name=>"index_unique_epics_on_issue_id", :algorithm=>:concurrently})
main:    -> 0.0048s
main: -- add_index(:epics, :issue_id, {:unique=>true, :name=>"index_unique_epics_on_issue_id", :algorithm=>:concurrently})
main:    -> 0.0019s
main: == 20231214095546 AddIssueIdToEpics: migrated (0.1871s) =======================

Rollback

r db:rollback:main
main: == [advisory_lock_connection] object_id: 182660, pg_backend_pid: 43601
main: == 20231214095546 AddIssueIdToEpics: reverting ================================
main: -- remove_column(:epics, :issue_id)
main:    -> 0.0023s
main: == 20231214095546 AddIssueIdToEpics: reverted (0.0053s) =======================

main: == [advisory_lock_connection] object_id: 182660, pg_backend_pid: 43601

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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