This fixes a number of documentation typos and bad references throughout

the codebase. Most are fairly simple fixes, though there's one reference

to the Django docs that no longer exists and had to be removed.
Some of this involves removing some duplicate docs (common when in both

an "Attributes" section and defined in a property or instance attribute

typing). There's also a move to Django's cached_property and away from

our old copy, which fixes another class of documentation issues.
There are still a few doc warnings that come up, having to do with

Django references. These will require fixes in beanbag-docutils.

However, there aren't many, and by and large this cleans up a whole lot

of our docs build output.

Built the docs. Saw that the only errors I hit were the ones that need

fixing in beanbag-docutils.