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.
WIP: update SlotAsset for slot abstraction

Mostly, though getThumb is still broken.
WIP: slot abstraction, checkpoint
Merge branch 'master' into unslot

Conflicts:
	app/models/asset.scala
	app/models/slot_asset.scala
	app/store/asset.scala
WIP: slot abstraction, checkpoint
Merge branch 'master' into unslot
WIP: slot abstraction, SlotAsset now building
WIP: slot abstraction, some work on tags

A few more SQL utilities, still broken...
WIP: slot abstraction, tag work checkpoint
Eliminate redundant implicit SQLTypes from Columns
WIP: slot abstraction, checkpoint
Change all Option.fold[Boolean] to better idioms

Cleanup only.  No effect.
Remove some interim angular content, view slot abstraction
WIP: slot abstraction, build successful!

Still completely broken (probably).
WIP: slot abstraction, generally working evolution

Passes basic checks; not yet tested on real dataset.
Slot abstraction: some small SQL fixes

Things are generally working.  A bit more testing, then this may be
ready.
Slot abstraction: minor rework of Slot and Selectors

Remove ObjectSelector; standardized fixed, but try to disentangle target
table from inheritance (esp wrt Slot and Container).  Minor schema and
evolution updates.
Standardize more model column/row functions