Fix User Creation Bugs

[axelstudios]

What's this PR do?

Fixes multiple bugs:

1. It was not possible to use the Admin Create a user form when also specifying a new organization to create
2. The same Create a user form failed to update the organization list after creating a new organization
3. The create_default_user script failed to add new users to the specified organization if it already existed, leaving the new user orphaned

How should this be manually tested?

1. Check that the Create a user form works as expected when specifying a new organization name, and then check the organization dropdown at the top right for the newly created org
2. Check that an organization owner (non-superuser) can still invite new members to their organization
3. Run manage.py create_default_user, specifying and existing organization name, and check that the new user was added to the organization as an owner

What are the relevant tickets?

#4095

[kflemin]

this worked for me!