Add support for attaching epics to an epic

What does this MR do?

This MR adds support for attaching Epics to an Epic and establish a parent-child relation ship.

Backend

parent_id was added to an epic

A new controller EpicLinksController was added. This controller uses the EpicRelations concern.

• create - calls newly created EpicLinks::CreateService (inherited from `IssuableLinks::CreateService)
• index - calls newly created EpicLinks::ListService (inherited from `IssuableLinks::CreateService)
• destroy - calls Epics::UpdateService because we only want to set parent to nil for an epic

Maximum 5 levels of epics are allowed, this is checked inside EpicLinks::CreateService.

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
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• 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?
• Security reports checked/validated by reviewer

Closes #8333 (closed)