Bugfix in YouTube ID pattern validation
Use the same regexp in the backend and the frontend
Merge branch 'master' into feature-assets
Move send_cancellation_email to assets/utils.py
Added is_modification to is_bookable
Implemented get_suitable_begin_times
Removal of pending reservations which not match with the slot duration if it is changed
Improve detecting reservations which are incompatible with slot duration

- A new method was created to detect the incompatible reservations.
- Slot duration being 1 (so no need to check the reservation time) or 60 being
a multiple of the slot duration (so no need to check the hour) are handled.
Remove incorrect underscore when calling to get_reservations_not_compatible_with_slot_duration
Added reservations whose durations are not multiple of slot_duration