Make js offset parsing stricter

Fixes #2927
Add position-min and -max options for ranges

Fixes #2928
Can use FascistCheckUser now that we have our own cracklib 2.9 on production
Fix segment pre-seeking

Was trying to make inContext do two different things, so separate them
(at least on js side) by type: Slot.inContext is slot,
SlotAsset.inContext is still constrained in asset (even in server-side
response isn't).
Upgrade to scala 2.11.4

To the extent possible here anyway