Create IssueEmailParticipant model

What does this MR do?

It adds issue_email_participants table and model for that table (including a new factory and spec file).

This model will be used for storing email participants for issues - they will be added or removed using quick actions.

The needed information are issue association and email address.

Migration output

rails db:migrate
== 20200922052316 CreateIssueEmailParticipants: migrating =====================
-- table_exists?(:issue_email_participants)
   -> 0.0009s
-- create_table(:issue_email_participants)
   -> 0.0088s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE issue_email_participants\nADD CONSTRAINT check_2c321d408d\nCHECK ( char_length(email) <= 255 )\nNOT VALID;\n")
   -> 0.0005s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- execute("ALTER TABLE issue_email_participants VALIDATE CONSTRAINT check_2c321d408d;")
   -> 0.0005s
-- execute("RESET ALL")
   -> 0.0002s
== 20200922052316 CreateIssueEmailParticipants: migrated (0.0223s) ============

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.

Closes #225293 (closed)