Starting in Djblets 0.10, extension hooks are expected to override the

initialize method instead of __init__, and no longer need to call

the parent shutdown. This was intended both to slim down the code for

hooks and to allow for dynamically disabling/re-enabling hooks and to

optionally start hooks in a disabled state.
This change updates all of our hooks to use the new initialize and

shutdown behavior, and to make use of base registry hooks where

possible. It also enhances the documentation, providing better guidance

for the kinds of values that each hook expects.

Unit tests pass.
Tested that my installed extensions didn't outright break.