We patch User with custom methods useful within Review Board. Since

this is just monkey-patching, we don't have a good way to convey the

presence of the methods for any code importing

django.contrib.auth.models.User.
To address this, we're now changing the way we do the patching. All the

methods now live on an object that gets mixed in to a new User class

when type-checking, but is otherwise assigned as normal at runtime.
It's still the same User object at runtime. The only impact is for

type checking. To benefit from this, going forward we'll want to import

reviewboard.accounts.models.User (AnonymousUser is also provided for

convenience). This will ensure our typed class is used for type hints.

This can be done incrementally as needed, and won't introduce any kind

of compatibility challenges.
It's worth noting that Django does have an ability to swap out the

User model with a custom one, but this doesn't get us what we need.

Anything importing the old User model will continue to get that model

and not ours, and would introduce significant compatibility challenges,

so it's not an option.

All unit tests pass.
Tested updating a few modules making use of these methods and seeing that

the typing was correct.