Make it easier to use 'WIP' in merge requests

See the discussion here: https://about.gitlab.com/2016/01/08/feature-highlight-wip/

Right now, to add or remove WIP, you have to edit the merge request title. People suggested alternative solutions:

• make a label 'WIP'
• make it a switch with a button (a db field of a MR)
• ?

Options that are requested:

• make it easier to add / remove (not having to edit the title)
• make it possible to filter for it (right now, you can already search for WIP, which works)

I'm open for suggestions to make it easier to use this. However, the current implementation already allows you to filter for it. Adding and removing the WIP state could be easier, but adding further UI complexity is not welcome.

@creamzy @dzaporozhets let me know if you have any thoughts about this.