Bump django-bootstrap to version 5
Exchange bootstrap files (js & css)
Adapt settings
Migrate to new version/resolve breaking changes
Move layout adaptions previously included in bootstrap.css into custom.css

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

[refs kif/akplanning#102]