Filter events for users by participant type.

Events returned for a user can now be filtered by participant_type. JSON api doc will be updated after the pull request is merged.

Example:
http://localhost:8080/api/me/event?participant_type=ORGANIZER
or
http://localhost:8080/api/user/{user_key}/event?participant_type=ORGANIZER


**Datastore implications:**
Requires re-creating events or editing all events (which forces indexing of all properties). The newly indexed fields in the event object will not be available for searching otherwise.