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