Remove a reservation if it's incompatible with its AssetAvailability.

When there's a change in an AssetAvailability instance, all reservations which
would not be allowed according to the new availability will be cancelled.
Handle assets slot time granularity

New settings option ASSET_SLOT_GRANULARITY will set the minimun possible time
which can be assigned to slot_duration.

The slot duration of each asset will be rounded to the nearest multiple of
ASSET_SLOT_GRANULARITY.
Send emails to users when a reservation is cancelled because an AssetAvailability has changed