Add support for closing epics

What does this MR do?

This MR adds support for closing Epics by introducing a close button on UI of Epic page.

Epic Status Toggle

Backend

• columns state, closed_by_id and closed_at were added to the Epic model
• state machine was added to the Epic. It is the same as for Issue
• for closing and reopening issues Epics::CloseService and Epics::ReopenService are used
• it adds quick actions for closing/reopening epics
• it adds API support for changing the state of epics

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?

CE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21771

Closes #7013 (closed)

Closes #3678 (closed)

Closes #7340 (closed)