Move send_cancellation_email to assets/utils.py
Implemented get_suitable_begin_times
Modified is_bookable to handle reservation modifications

Function is_bookable was modified to allow an old reservation as a parameter.
If that parameter is specified, it's not checked if the user is exceeding the
maximun number of reservations and that reservation is excluden when checking
collisions.
Removal of pending reservations if they don't match with slot duration when assets change

When an asset is changed, reservations which do not begin at a suitable time or
whose duration is not a multiple of slot_duration will be cancelled.
Added function to send an email when an asset reservation is modified

Email template for reservations modifications added as well.
Spanish translation for emails of reservation cancellations and modifications