Add missing migration for Partners & AssignedReviewers

[wes-otf]

Related to #4790. I missed an edge case where an AssignedReviewer was also a Partner, in which the migrations would fail as the type attribute is protected in the model. This moves all types of partner to reviewer.

Test Steps

• Ensure AssignedReviewers with type of partner get migrated to reviewer and that all migrations pass successfully.

[frjo]

Running this:

git switch main
uv run python manage.py migrate
git switch fix/add-assignedreviewers-migration
uv run python manage.py migrate

I get this error:

django.db.migrations.exceptions.InconsistentMigrationHistory: Migration users.0028_remove_partner_group is applied before its dependency funds.0134_change_partner_assigned_reviewers_role on database 'default'.

[frjo]

I think we can make changes to 0028_remove_partner_group.py‎ directly.

The few system that have applied it likely do not have this edgecase so do not need fixes.

[frjo]

See comments above.

[wes-otf]

Yeah I had to manually remove 0028_remove_partner_group from the database to get this in but this makes a little more sense, I think it should be good to go!

[frjo]

@wes-otf The new function was never run so I did that minor fix. Also fixed so apps.get_model() always was used.

Works well in my testing now.

I think we can do a new release when this is merged in.