Add multi-version API support via media types
API v3:
- Media type: application/vnd.de.thm.arsnova.v3+json
- Serializes date as strings conforming to ISO 8601.
API v2:
- Media type: application/vnd.de.thm.arsnova.v2+json
- Serializes dates as timestamps including nanoseconds.
An indentation setting has been added: api.indent-response-body:false
.