User: make username case insensitive (and save original styling).

We want 'User' and 'UsEr' to mean the same user. Apparently that's not the default in
django. This normalizes the user to ensure we enforce this.

myauth/models.py

@@ -1,4 +1,4 @@
-from django.contrib.auth.models import AbstractUser
+from django.contrib.auth.models import AbstractUser, UserManager as DjangoUserManager
 from django.core import validators
 from django.db import models
 from django.utils.deconstruct import deconstructible
@@ -15,9 +15,16 @@ class UnicodeUsernameValidator(validators.RegexValidator):
     flags = 0
 
 
+class UserManager(DjangoUserManager):
+    def get_by_natural_key(self, username):
+        return self.get(**{self.model.USERNAME_FIELD + '__iexact': username})
+
+
 class User(AbstractUser):
     username_validator = UnicodeUsernameValidator()
 
+    objects = UserManager()
+
     username = models.CharField(
         _('username'),
         max_length=150,
@@ -28,3 +35,7 @@ class User(AbstractUser):
             'unique': _("A user with that username already exists."),
         },
     )
+
+    @classmethod
+    def normalize_username(cls, username):
+        return super().normalize_username(username).lower()