This introduces BaseRegistryMultiItemHook, which is similar to

BaseRegistryHook but handles registering/unregistering a list of items

at once. It's useful for extension hooks that otherwise need to manage

multiple items, helping to consolidate that logic.
Unlike existing hooks in Review Board that manage multiple items, this

is also careful to unregister any items that were registered upon

encountering an error, so that some items don't remain permanently

registered even if the extension is shut down.
Unit tests were added for this hook.

Unit tests pass.
Moved some extension hooks to this in Review Board and ran tests. Those

also pass.