This commit alters views for the following models to use the markdown cache if
present:
* AbuseReport
* Appearance
* ApplicationSetting
* BroadcastMessage
* Group
* Issue
* Label
* MergeRequest
* Milestone
* Project
At the same time, calls to `escape_once` have been moved into the `single_line`
Banzai pipeline, so they can't be missed out by accident and the work is done
at save, rather than render, time.
Safari 9.0 does not yet honor the HTML5 `origin-when-cross-origin` mode,
and it's possible load balancers/proxies strip the HTTP_REFERER from
the request header. In these cases, default to some default path.
Closes#3122
Closes https://github.com/gitlabhq/gitlabhq/issues/9731
