* Upgrade Django to 3.0 and other dependencies to recent versions
* Fix otp version contsraint
* Remove six dependency
* Resolve some warnings
* Fix failing tests
* Update django-countries
* Resolve all RemovedInDjango31Warnings in test suite
* Run isort
* Fix import
* Update PostgreSQL version on travis
* Drop support for maindomain_urls/subdomain_urls in plugins
* Allow to use a custom domain per event
* Fix bug when manually saving domains
* Fix custom domains in debugging
* Fix middleware
* Fix middleware again, update docs