Enable usage of django.contrib.messages.
The Django messages framework has improved quite a bit since the early

days, and is now a useful thing to have in the admin UI. It already

stored cookies with these messages, but they just built up and were

never used or cleared. By activating the context processor, the admin UI

can provide much nicer responses to actions, such as deleting objects.
We can probably make use of this in some good ways going forward as

well.
While activating this, I noticed that output in the admin UI was wrong.

Our HTML was quite broken, since we never tested it before. This has

been fixed.

Deleted some objects and saw the better errors.
Note that, since these have been saved over time, you may see a bunch of

stale messages the first time you visit a part of the admin UI.