Add multiple email participants via a single quick action [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Adds the /invite_email quick action (with preview as per designs). When used, adds a system note and the email_participant row in the db

This is behind a feature flag issue_email_participants

Closes #225309 (closed)

Screenshots (strongly suggested)

Migrations

lee@cc-gdk-2:~/gitlab-development-kit/gitlab$ bundle exec rake db:migrate:up VERSION=20201209163113 RAILS_ENV=development
== 20201209163113 RecreateIndexIssueEmailParticipantsOnIssueIdAndEmail: migrating 
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:issue_email_participants, "issue_id, lower(email)", {:unique=>true, :name=>"index_issue_email_participants_on_issue_id_and_lower_email", :algorithm=>:concurrently})
   -> 0.0022s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- add_index(:issue_email_participants, "issue_id, lower(email)", {:unique=>true, :name=>"index_issue_email_participants_on_issue_id_and_lower_email", :algorithm=>:concurrently})
   -> 0.0029s
-- execute("RESET ALL")
   -> 0.0002s
-- transaction_open?()
   -> 0.0000s
-- indexes(:issue_email_participants)
   -> 0.0014s
-- remove_index(:issue_email_participants, {:algorithm=>:concurrently, :name=>"index_issue_email_participants_on_issue_id_and_email"})
   -> 0.0012s
== 20201209163113 RecreateIndexIssueEmailParticipantsOnIssueIdAndEmail: migrated (0.0090s)

lee@cc-gdk-2:~/gitlab-development-kit/gitlab$ bundle exec rake db:migrate:down VERSION=20201209163113 RAILS_ENV=development
== 20201209163113 RecreateIndexIssueEmailParticipantsOnIssueIdAndEmail: reverting 
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:issue_email_participants, [:issue_id, :email], {:unique=>true, :name=>"index_issue_email_participants_on_issue_id_and_email", :algorithm=>:concurrently})
   -> 0.0020s
-- execute("SET statement_timeout TO 0")
   -> 0.0001s
-- add_index(:issue_email_participants, [:issue_id, :email], {:unique=>true, :name=>"index_issue_email_participants_on_issue_id_and_email", :algorithm=>:concurrently})
   -> 0.0032s
-- execute("RESET ALL")
   -> 0.0001s
-- transaction_open?()
   -> 0.0000s
-- indexes(:issue_email_participants)
   -> 0.0013s
-- remove_index(:issue_email_participants, {:algorithm=>:concurrently, :name=>"index_issue_email_participants_on_issue_id_and_lower_email"})
   -> 0.0011s
== 20201209163113 RecreateIndexIssueEmailParticipantsOnIssueIdAndEmail: reverted (0.0085s)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #225309 (closed)