Add __all__ to reviewboard.extensions.hooks.

The hooks module has a bunch of its own classes, but also imports some stuff
into its namespace (which is then expected to be exposed to the world). This
change adds __all__ to said module in order to clarify what should be exported
(which also makes pyflakes happier).

Testing done:
- Ran pyflakes
- Ran unit tests

Reviewed at https://reviews.reviewboard.org/r/6389/
Merge branch 'release-2.0.x'