extract event_dates from events

- an event has more than one event date now
- event date encapsulates start, end, and location of the event
ensure basic functionality after refactoring
add validation in EventDate: end_at must be after start_at
display addtional dates for events on start page
add additional description to event/concert forms, display additional info on start page
display additional info on event detail page
show multiple dates on concert detail page
fix rehearsal plan
fix calendar feed
remove some warnings
fix concert list
Merge branch 'feature/zusatztermin'