Set a maximum of one AssetAvailability for each KQ

AssetAvailability.kq was changed to OneToOneField in order to prevent a KQ
having more than one AssetAvailability.

Field asset_availability was added to both KnowledgeQuantumResource and
PrivateKnowledgeQuantumResource.
Displaying asset availability in classroom view

Backbonew model KwnowledgeQuantum was modified to handle asset availability.
Classroom view was modified to show asset availability in unit list