[Feature]: Remove username field from user model

[brylie]

Feature Description

Remove the username field from the user model and use the email address as username.

Motivation

We don't have a use case for usernames.

[hudy0]

Hello, @brylie I was working on this issue.

Issue

The task is to eliminate the username field from the user model while retaining the ability for users to log in using either a username or an email address.

Changes Made

Email Backend

UserModel = get_user_model()
class EmailBackend(ModelBackend):
    def authenticate(self, request, username=None, password=None, **kwargs):
        try:
            user = UserModel.objects.get(
                Q(email__iexact=username),
            )
        except UserModel.DoesNotExist:
            UserModel().set_password(password)
            return
        except UserModel.MultipleObjectsReturned:
            user = (
                UserModel.objects.filter(
                    Q(email__iexact=username),
                )
                .order_by("id")
                .first()
            )

        if user.check_password(password) and self.user_can_authenticate(user):
            return user

LoginForm

class LoginForm(AuthenticationForm):
    username = forms.CharField(label="Email / Username")

User Model

class User(AbstractBaseUser, PermissionsMixin):
    email = models.EmailField(_("email address"), unique=True)

LoginView

class LoginView(auth_views.LoginView):
    form_class = LoginForm
    template_name = "registration/login.html"

These changes have been tested and if are compatible with the project's requirements.
I will submit the Pull request

[brylie]

It looks good so far. Go ahead and fork this repository, commit your changes to your fork, and open a pull request. That way we can review and discuss the code. It will also make it easier to give you credit for your contributions.