Add lower and upper Orderings for Range
Schema for complete slot abstraction

Make the slot table abstract, remove ids, replace with inheritance
(which just functions as simple inclusion and denormalization of the
remaining columns).

WIP: No code, no views, no evolutions, broken.
WIP: Basic slot abstraction models

Remove all mention of Id from Slot models, and rename AbstractSlot to Slot.

Other models not updated, no evolutions, broken.
WIP: full slot abstraction

More on Slot itself, some on Comment, etc.