Improve the documentation and logging for the Active Directory backend.
Improve the documentation and logging for the Active Directory backend.

This fleshes out the docs for Active Directory, helping to make some
parts a lot more clear for future maintenance work. These docs cover the
details of function arguments and return types, as well as Django
settings and their siteconfig equivalents.

Some older code has also been cleaned up a bit, better conforming to
current coding styles and simplifying logic.

The backend also now has its own logger, which allows us to simplify
the various logging calls and make them more useful.

Testing Done:
Unit tests pass.

Built the docs and checked through them for reference errors, typos, and
build errors.

Reviewed at https://reviews.reviewboard.org/r/9992/

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