This is a large patch that refactors the action registry system

implemented by a past student to use a proper registry. Much refactoring

has been done:

Action base classes now live in reviewboard.actions. This package

  also includes header dropdown actions.
Review request-specific action classes now live in

reviewboard.reviews.actions.
ALL action hooks support new-style action classes and old-style

  dicts. Previously, there was a discrepancy betweeen which hooks

  supported which type of action.
ActionHook is now deprecated in favour of ReviewRequestActionHook.

  The behaviour was the previously the same, except ActionHook supported

  old-style dicts instead of classes. Now that the behaviour of all

  hooks has been unified, we no longer need this distinction.
The {% child_actions %} templatetag has been moved from reviewtags

  to a new actions template tag library. It now takes the action it

  should list the child actions of.
The _DictAction and _DictMenuAction classes have been turned into

  the DictActionMixin.
There are now two action registries: one for review request actions

  and one for header actions. This allows us to keep them seperate.
We now store instances of dict actions instead of the dicts

  themselves. This is because you cannot store a dict in a set (as

  it is unhashable) and therefore cannot be put into a Registry.
Generic action templates now live in templates/actions.

Ran unit tests.
Built the docs and looked through them.