Split up extensions/base.py into several files.
Back when the extension support was smaller, everything went into

base.py. This become harder to work with as things grew, so now it's

split up into multiple files.
The orignal base.py still exists, and imports the main classes that were

used previously. It's there for backwards-compatibility, but will be

removed in a future version.

Unit tests pass. (There was a failing test before this change, which I've fixed.)
Tested against Review Board as well, which passes (except one bad import in a tests.py,

which imported something imported by base.py, and really lives in models.py, and is

a Review Board bug).