Disables sorting when guests are viewing

Scoped labels in EE require additional changes in CE code.

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.

* Add Labels tab to Groups
* Add decorator for label so it's aware of Milestones.