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

currently only works when creating a new owner

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

avoid clashes with python keyword

allow AK owners to be empty
infer AK wish from empty owner list
show wish status in Admin overview

This message is used by the mixin to find the corresponding event for a slug

Create mixin (FilterByEventSlugMixin) to handle even slug parsing in generic views
Add submission url config
Include submission urls in central url config
Add index view for submissions

Create html template
Add translation
Add property for easy access to list of AK owners to AK model

Resolve first partial task of #7

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

create AK Types
create AK Tracks
create AK Tags
create AK Requirements
create AKs