Add or complete docstrings
Remove code smells
Disable irrelevant warnings
Update translations to changed line numbers and line breaks
Move duplicated code for event field pre-population and event timezone adaption to mixins

This used the common fullcalendar v5 instance instead of a special v3 version only for this editor
Improve visualization of availability deletion
Improve (event specific) timezone handling and remove dependency to moment-timezones

This implements #129

availabilities in AK submission
availabilities in AK wish submission

avoid clashes with python keyword

add model class for availabilities
register availability model for admin view
add availability migration
add German translations for availability model
remove availability modelling TODOs