Allow to authenticate by email field

[mlen108]

There should be a way to explicitly pass/use email field in
https://github.com/incuna/incuna-auth/blob/master/incuna_auth/backends.py#L13

It raises error if User model does not contain username field and user tries to log in with just "account_name" (note missing "@").

[ghickman]

@mattack108 – Are you getting this with a 1.5 custom user model?

[mlen108]

@ghickman YES. When I try to login to /admin

[ghickman]

@mattack108 – Use the default Django one and it should be fine.

I think we need to deprecate this backend for 1.5+ because of this.