Issue #20: Lock Events

Lock Events so that other events can't push them.

- Add lock field to django Event model
- Update factory with a default for new lock field
- Create get_next_available_unlocked, a method for EventManager that
  returns a (start, end) of the next available slot that doesn't have
  locked events.
- Add unit tests for EventManager.get_next_available_unlocked
- Add lock choice to backbone EventEdit view
- Add lock ui to backbone Event template