Order slots by room order first

* lca2018 has a situation where we have multiple slots starting at the
same time, but ending at different times
* The headers of the timetable grid are sorted by room sort order
* In sqlite at least, ordering by start,order seems to implicitly
resolve duplicate start times by looking at the other sort fields
first, and will only sort on order if all other fields are identical

* This results in the slot that ends first going in column 1, which
gets out of sync with the room listed in the header

* I can't figure out how to solve this in the database, so...

* Force the slots to be sorted by room order.
* Then, for each start_time, select out slots starting at that time
and operate on them
* This both gets the slots in the right order *and* keeps multi-room
slots with the right colspan. Yay!

* It's possible that this wouldn't be needed on some DBs which might
do the sorting differently.