It toggles the 'WIP' prefix in the MR title.

Now a merge request with a blank description will no longer produce a
merge commit message like this:

```
Merge branch 'foo' into 'master'

Bring the wonders of foo into the world

See merge request !7283
```

What an improvement! 🎉

Closes #22438

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6414#note_15765156

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6414#note_15750344

So that we could just use it in testing for MergeRequest#all_pipelines

This way we could also show pipelines from commits which
were discarded due to a force push.

Signed-off-by: NRémy Coutable <remy@rymai.me>

Add test for closed MR without fork

Add view test visibility of Reopen and Close buttons

Fix controller tests and validation method

Fix missing space

Remove unused variables from test

closed_without_fork? method refactoring

Add information about missing fork

When closed MR without fork can't edit target branch

Tests for closed MR edit view

Fix indentation and rebase, refactoring

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>