Merge branch '443-events-api-for-calendarjs-no-longer-accisible-for-unauthorized-users' into 'master'

Resolve "Events api for calendarjs no longer accisible for unauthorized users"

Closes #443

See merge request !499

Fix intereference between NGINX basic authentication and DRF
http basic authentication.

Fix event dates in plaintext newsletters

Closes #435

See merge request !493

Introducing: API Versioning

See merge request !491

Thaliapp: members api

See merge request !490

ThaliApp: events api

See merge request !489

Make sure you can only access other (=queued/cancelled) registrations when you have the perms to do so

Introducing: API Pagination

See merge request !492

Fine no longer required for events without registration

Closes #437

See merge request !487

This is now checked only if the event status is REGISTRATION_REQUIRED in the clean() of the model.