Add new property to model to simply access planned durations of AK

Add contact email address to event model
Show email link in footer
Fix missing event reference in AKDetailView

Add base url field to event
Generate Wiki link from title and base url

Improve blank and unique constraints of model:
- allow blank starting times of slotes to store durations before scheduling and adapt string representation
- change unique constraint from name and shortname of AK to unique for each event only
Implement handling of tags for submission and update (auto-creation) by using a semicolon-separated text file instead of multiple choice field
Implement handling of durations for submission
Automatically generate short name
Use different forms for submission and update
Use special adapted template for AK updates instead of submission template
Fix EventSlugMixin for POST requests

Add description to category model
Show tabbed list of different categories of AKs on submission overview

avoid clashes with python keyword

place in event not required
track in AK not required
building and properties in room not required

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