Allow header auth for kosync endpoint

[jbergler]

Sort of related to #479, this endpoint uses a different auth path than the rest of the app, and the logic to check the header auth if configured is missing. Add it.

[jbergler]

@crocodilestick you might have other opinions on this, but I wonder if the right solution here is to re-use more of the logic from usermanagement.

I get the feeling that @user_login_required isn't the right decorator for these endpoints, but it doesn't seem too complicated to replace the auth in this file with the shared logic in some way. Maybe it's a version of requires_basic_auth_if_no_ano without the guest logic?

Certainly for now, this PR feels like the quick fix, I've tested it on my instance and it solves my problem.

[poupounetjoyeux]

Thanks for your fix @jbergler, I was looking for the exact same thing !

Following your last comment I agree with you that having the same logic everywhere using the requires_basic_auth_if_no_ano without guest logic could be really nice

Hope the fix will be merged soon

[poupounetjoyeux]

Following this, I also propose the #1004 using base decorators
Tested on my side and working without issue

[crocodilestick]

Hey @jbergler , sorry this took a while to get merged, great work though!