When rendering a label we want to check 'scoped_label' feature
availability on a project/group where label is being used. For
this reason a label presenter is used in UI and information about
context project/group is passed to this presenter.
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.