You now register logintype->callback rather than
having check_auth and check_password as the two
only possible functions.

modules now must provide the login types they
support when they register, instead of providing
a function get_supported_login_types.

The auth handler is initialised before the modules
are loaded and so supported login types must be
worked out later

A load_legacy method wraps old-style modules so
that they work with the new system

Also renamed the callback types to the form
of XYZ_CALLBACK

(could previously return either a string or a tuple)

Reworded some stuff
Added a docstring
Added a newline in docs
Added some spaces so string concatinate as expected
Made mypy '# type: ignore' comments more selective

…h_providers_to_new_module_system

…h_providers_to_new_module_system

…le_system

The documentation mentions the username parameter of auth checkers as
`user` even though the code doesn't, in order to make it clear this is
the same value clients include in login requests as `user`.