Refactor FallbackToAuthBasicMiddleware to non-deprecated middleware syntax

[mikkonie]

I guess we adapted this from some very old bit of code as it uses the deprecated middleware style from Django <1.10. It still works, but will probably be dropped eventually.

See here for how we're supposed to implement these nowadays.

Not urgent and I don't want to spend time on this now since #1999 has already taken too long. I'll tag this when next working on something relevant.