Updated users/admin.py with fix for issue #146 to allow user creation through the Django admin

{{cookiecutter.repo_name}}/{{cookiecutter.repo_name}}/users/admin.py

@@ -1,13 +1,27 @@
 # -*- coding: utf-8 -*-
 from django.contrib import admin
+from django import forms
 from django.contrib.auth.forms import UserCreationForm, UserChangeForm
 from django.contrib.auth.admin import UserAdmin as AuthUserAdmin
 
 from .models import User
 
 
+class MyUserCreationForm(UserCreationForm):
+    def clean_username(self):
+        username = self.cleaned_data["username"]
+        try:
+            User._default_manager.get(username=username)
+        except User.DoesNotExist:
+            return username
+        raise forms.ValidationError(self.error_messages['duplicate_username'])
+
+    class Meta(UserCreationForm.Meta):
+        model = User
+
+
 class UserAdmin(AuthUserAdmin):
-    create_form_class = UserCreationForm
+    add_form = MyUserCreationForm
     update_form_class = UserChangeForm