* Add HTTP header authentiation backend/middleware
* Add docs for remote header auth
* Improve docs on external auth
* Add warning for unknown REMOTE_USER_HEADER_TYPE
* Move active check for header auth to middleware
Add extra header type sanity check to the backend
* Add test cases for remote header login
* Improve header-based authentication
- remove the 'ID' mode
- add CustomHeaderBackend to AUTHENTICATION_BACKENDS conditionally
- rewrite CustomHeaderBackend and CustomHeaderMiddleware to
use less inherited code
- add more test cases
Co-authored-by: Pēteris Caune <cuu508@gmail.com>
