Actor filtering and other improvements

@Wilduck This PR provides the following:

- Events can now be filtered by actor
- Event filters are accepted as **event_filters instead of explicitly listing all of the same possible filters for each of the 3 primary methods
- The expiration time defaults to datetime.max and is no longer nullable. This allows the query to be simpler for expiration checking. Proper data migrations were created
- DB indices added to expiration time and time fields
- Filtering for subscriptions was change so that subscriptions with only_following=False are handled in a more optimized manner (and __in query is used for those subscriptions rather than lots of OR queries)