The reviewboard.extensions.base module was forwarding the
Extension/JSExtension classes from Djblets. This basically meant we had
a copy of the class, rather than a version specific to Review Board. In
practice, that's not so bad, but it also meant that we couldn't have
documentation specific to Review Board, and we also weren't properly
carrying over documentation from Djblets (due to "#:" lines not carrying
over).

We now have local subclasses instead, which we can document and use to
point back to the parent class documentation.

The rest of this file has also been documented.

Unit tests pass.
Existing extensions still work.
Built and viewed documentation.