Handle more reservations restrictions

More restrictions were added to models:
 - Courses have a maxium number of reservations and of pending reservations
allowed.
 - Assets have a minimun time in advance for reservations and cancelations.

cancel_reservation and is_asset_bookable were modified to take into account the
new restrictions.
Add fields max_reservations_pending and max_reservations_total to AssetAvailabilityResource

These fields will always have the value which is stored in the Course of the KQ
of the availability.

They have been added to AssetAvailabilityResource to simplify showing this
information in the classroom view.

Backbone models have been modified accordingly.
Display more requisites for making reservations when viewing assets in classroom view.

Maximun allowed number of reservations (both pending and total) and required
time in advance for reserve and cancel reservation have been added.