When populating the UI, we fetch actions based on various attachment

points. When we do this, the registry has to iterate through every

action every time, returning each one matching the attachment point.

This is algorithmically slow, and only gets worse as we expand usage of

actions.
Now, when registering an action, the action gets placed in a cache of

attachment points to actions. Unregistering will remove that action from

the cache. This makes lookup very fast.
There's also a bit of future-proofing thrown in for upcoming support for

actions that attach to multiple attachment points (part of a larger set

of work for enhanced actions in Review Board 7.1).

Unit tests pass.
Tested various parts of the UI, making sure the right actions are in

the right places.