R1:85bf88e400fa

Allow pastes to be flagged

Summary:
This does a few things:

- Allows you to flag pastes. This is straightforward.
- Allows Applications to register event listeners.
- Makes object action lists emit a 'didrenderactions' event, so other applications can add more actions. The Flags application injects its action in this way. This should generally make it much easier to add actions to objects when we add new applications, with less code duplication and better modularity. We have a really hacky version…