Promote an issue to an epic

What does this MR do?

It adds support for Promotion Issues to Epics. Issue to Epic promotion is handled by EpicsIssuePromoteService. Now only promotion using quick commands is supported. System notes about promotion generated for issue and epic are included as well.

• We display a warning when using this quick command.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #3777 (closed)

Closes #7729 (closed)