Refactor model layout

src/registrar/models/user.py

@@ -0,0 +1,17 @@
+from django.contrib.auth.models import AbstractUser
+
+
+class User(AbstractUser):
+    """
+    A custom user model that performs identically to the default user model
+    but can be customized later.
+    """
+
+    def __str__(self):
+        # this info is pulled from Login.gov
+        if self.first_name or self.last_name:
+            return f"{self.first_name or ''} {self.last_name or ''}"
+        elif self.email:
+            return self.email
+        else:
+            return self.username