This caches the result of Appearance.first in a similar fashion to how
ApplicationSetting instances are cached. We also add some NOT NULL
constraints to the table and correct the timestamp types.

Fixes gitlab-org/gitlab-ce#36066, fixes gitlab-org/gitlab-ce#31698

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.

Updated tests

Added tests

Added awesomeeeeee icons

Added tests

Added awesomeeeeee icons

The only major difference with the EE version is the change from a light and dark logo to only a header logo
The dark logo wasn't used anyway, so it seemed to make sense to me to rename the field to the actual function of it

The logo is now rendered as pure SVG, rather than image referencing a
svg. The SVG has an id and the shapes of the logo have a shared class.
The shapes change their fill color on hover with a transition.

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

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

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

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

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