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
WIP
prefix (underline is visible because of hover)
Link to add
WIP
prefix
Link to remove
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