User can edit closed MR with deleted fork

## What does this MR do?
User can edit closed MR with deleted fork (can't change "Target branch"). When fork is deleted "Reopen" button is hidden.

## What are the relevant issue numbers?
Closes #19315 

## Screenshots (if relevant)
* hidden Reopen button and information about deleted fork
![Zrzut_ekranu_2016-08-12_o_13.19.24](/uploads/d288c5625e788382e31b2979acb601df/Zrzut_ekranu_2016-08-12_o_13.19.24.png)

* editable fields for closed MR without fork
![Zrzut_ekranu_2016-08-05_o_12.24.38](/uploads/1549e54d4bc2a9939ef296ce66139706/Zrzut_ekranu_2016-08-05_o_12.24.38.png)


## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

cc @ubudzisz @yorickpeterse @grzesiek @tmaczukin

See merge request !5496