This change simplifies some of the extension hook test setup code,

preparing for new hooks and unit tests. There's a new

BaseExtensionHookTestCase and ExtensionHookTestCaseMixin that

handles the main setup and teardown code for a test. This builds upon

the Djblets extension testing code, meaning we get all the benefits of

standardized and consistent extension management without ever having to

have tests specialize any of this.
There are some other small cleanups throughout for docstrings, or a little

bit of code or function names here and there. It's not a full revamp of

the code, just some tidying up.

Unit tests passed.