Makes the username configurable via settings & small tweaks to method args

[Chris-Thornton-Harvard]

Fixes a couple of args in methods that were expecting 2 positional arguments but were only receiving 1.

Adds support for an optional LTI_TOOL settings object that can have a use_person_sourcedid boolean.
If present and True, use the SIS ID of the user as the username otherwise the users sub value will be used.

[haydn9000]

👍

[cmurtaugh]

I ended up re-working the middleware and backend classes to be based on the auth classes from Django 4.2 instead of Django 5.2 and that created a bunch of conflicts with this branch. So - I just manually moved these changes over to the add_authentication_backend branch.

[cmurtaugh]

I also added a get_username(request) function; if someone wanted to use something other than the sub or person_sourcedid values, they could just subclass the middleware class and override get_username().