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)
![wipless_title](/uploads/72a6f7119ba9d8043ca8329641e97c3b/wipless_title.png)

## Link to remove `WIP` prefix
![wip_title](/uploads/8620ad65da9ef620b180603520fead55/wip_title.png)

## System note after WIP is added
![wip_sysnote](/uploads/2de073b75e854d2c9e243eb8b5d5c259/wip_sysnote.png)

## Widget with link to remove WIP
![wip_widget](/uploads/cf83ea93743c4c26d9df759c17cb9d7b/wip_widget.png)

## Flash after WIP is removed
![wip_flash](/uploads/27b7240cd5d7ceeb8b7b477abd94d7ff/wip_flash.png)

## System note after WIP is removed
![wipless_sysnote](/uploads/c0d3368abdf21a2f253532a9a9594d90/wipless_sysnote.png)

## Widget when current user cannot remove the WIP prefix
![wip_widget_unauthorized](/uploads/174ccf1674be86dc81c3078fe297acb7/wip_widget_unauthorized.png)

cc @creamzy 

See merge request !3006