Minor cleanups to model

* Add default ordering to TimeSlot
* Explain where the hell `room_count` comes from!
The prefetches - they do nothing!

`prefetch_related` is not behaving as expected

Removed `distinct()` because it won't do anything and seems to force an extra query
Further reduce number of queries made during Grid rendering

Relations aren't pre-filled in even if the object has been fetched from a reverse relation! Kinda dumb.

This commit works around this silliness.