Special case for "[Un]marked WIP" system note

Right now when a merge request is marked or unmarked as a Work in Progress, it generates a system note for the title change:

Title changed from **WIP Create milestones in the group** to **Create milestones in the group**

It'd be cool if we could handle a special case where, if we detect the only difference is the addition or removal of our WIP pattern, it generates a special system note instead of the title change one:

Unmarked as Work in Progress

@DouweM Maybe one for interviews?

cc @JobV

Added feature proposal label

Super minor, nice for an interview, indeed.

mentioned in merge request !3006 (merged)

Status changed to closed by commit 3f22a92f

mentioned in commit 3f22a92f

mentioned in commit 42c43041

Mentioned in commit pfjason/gitlab-ce@42c43041

Mentioned in commit pfjason/gitlab-ce@3f22a92f

mentioned in merge request !14785