Ignore capacity criterion if capacity is unknown
Don't show None capacity if it's unknown
Fix contextmanager default value not being falsy

This resulted in unexpected behavior such as access checks being
disabled because the code assumed it was rendering an offline website
even though it wasn't
Avoid ElementAlreadyInIndex when moving events

Someone might click the link that performs the move twice and in that
case the events are already in the target category. Those are now simply
skipped (if the source category is not the expected one).
Fix start date in calendar book link
Fix phone validation in regform
Properly set field as required
Fix typo
Fix date format when cloning/editing a booking
Properly send registrant email to organizer

Even if all emails to the registrant are disabled, the organizer should
still be able to get an email notification.
PEP8
Use correct time(zone) in alarm emails
Fixed wording of notification
Fix abstract fields not showing up in the book

* Also, show and hide them according to the settings
Fix event arrival dates when start_time > end_time
Timetable arrows shifted when less than 20 mins
Code simplification and visual improvements
Wording - 'Edition' -> 'Edit'
Nicer HTML color -> RGB conversion function

Properly handles 3-digit hex codes too.
PEP8/style fixes