Add repetition info on booking creation email
Prevent sending reminders on booking creation
Add get_month_start/end helpers
Add round_up_month helper
Add Room.is_in_digest_window
Add weekly reservation digest

Add email template and scheduler task.
Disable sending reminder for weekly occurrences.
Mark weekly occurrences created within digest window as notified.
English: 'booking at' -> 'booking under'
Add global setting to disable reminders
Add migration revision
Add per-room setting to disable reminders
Rename notifications to reminders in the interface
Disable room reminder inputs if globally disabled
Fix incorrect behavior when editing start time

 - set the start date before the duration which relies on the start date to
   assert its validity.
 - removes inconsistent error message about an incorrect end time which
   corresponds to the old start time plus the new duration.
 - fixes 1583