Use alerts to show the Django messages in the admin UI.

Review Request #10886 — Created Feb. 6, 2020 and submitted — Latest diff uploaded


Review Board


The admin UI makes use of the Django messages app to display when items
are successfully saved, amongst other things. We weren't styling these
anymore, since we dropped the default Django stylesheets and our
overrides file.

This change updates these to use the rb-c-alert component for messages
instead. This maps nicely to what messages provide, and is consistent
with the look of our UI.

Saved some objects and verified the alerts worked.