allow custom user models in rename_groups migration

[jamesdoh0109]

Fixed a bug in the rename_groups migration that prevented projects using custom User models from running migrations. The migration was originally hardcoded to use django's default auth.User model which caused an AttributeError when AUTH_USER_MODEL was set to a custom model.

When a project defined a custom User model via AUTH_USER_MODEL, the migration would crash with

AttributeError: Manager isn't available; 'auth.User' has been swapped for 'myapp.User'

This occurred because the migration directly referenced apps.get_model("auth", "User") instead of using the AUTH_USER_MODEL setting. This means any project using a custom User model could not run migrations.

As a solution, we updated accounts/migrations/rename_groups.py to properly support both default and custom User models:

Use AUTH_USER_MODEL setting

   # Before
   User = apps.get_model("auth", "User")
   
   # After  
   User = apps.get_model(settings.AUTH_USER_MODEL)

Add swappable dependency

   dependencies = [
       ("accounts", "0001_initial"),
       migrations.swappable_dependency(settings.AUTH_USER_MODEL),  # Added
   ]

This works because:

• settings.AUTH_USER_MODEL returns "auth.User" by default or the custom model path (e.g., "myapp.User")
• apps.get_model() accepts both formats: "app.Model" and ("app", "Model")
• swappable_dependency() ensures the migration runs after the User model is created, regardless of which app defines it

The changes are also fully backwards compatible:

• Projects using Django's default User model see identical behavior
• When AUTH_USER_MODEL is not set, it defaults to "auth.User"
• The migration ordering is unchanged for default User configurations
• No data migrations or database changes required

Tests:
Added test coverage in accounts/tests/test_migrations.py:
✅ Groups renamed correctly with platform_ prefix
✅ User group memberships updated properly
✅ All required platform groups created
✅ Migration uses AUTH_USER_MODEL setting (core fix)
✅ Handles users without platform groups
✅ Idempotent (safe to run multiple times)
✅ Works with empty database
✅ Compatible with both dotted string and split formats

[ryantanen]

LGTM