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.

Standardize the way we check for and display form errors

- Some views had a "Close" button. We've removed this, because we don't
  want users accidentally hiding the validation errors and not knowing
  what needs to be fixed.
- Some views used `li`, some used `p`, some used `span`. We've
  standardized on `li`.
- Some views only showed the first error. We've standardized on showing
  all of them.
- Some views added an `#error_explanation` div, which we've made
  standard.

See merge request !3531

- Some views had a "Close" button. We've removed this, because we don't
  want users accidentally hiding the validation errors and not knowing
  what needs to be fixed.
- Some views used `li`, some used `p`, some used `span`. We've
  standardized on `li`.
- Some views only showed the first error. We've standardized on showing
  all of them.
- Some views added an `#error_explanation` div, which we've made
  standard.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

- Add label description
- Show label row when filtering issues or merge requests by label