Easily (un)mark merge request as WIP using link
Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/3768 and https://gitlab.com/gitlab-org/gitlab-ce/issues/3516
## Link to add `WIP` prefix (underline is visible because of hover)

## Link to remove `WIP` prefix

## System note after WIP is added

## Widget with link to remove WIP

## Flash after WIP is removed

## System note after WIP is removed

## Widget when current user cannot remove the WIP prefix

cc @creamzy
See merge request !3006