Add start and end inputs below the calendar widget that can be used to add new entries to the calendar.
Inputs are based on html5 widgets, will fallback to simple text inputs in very old browsers. Modern browsers also provide an integrated validation of the time ranges. In older browsers, this will be done latest on server side.
This implements #124

This implements #174

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

availabilities in AK submission
availabilities in AK wish submission