fixes #4641 Ensure default user role only after we save/update.

## Solution

Use `after_save` callback to save a user role.
Add default user role for users who don't have a role assigned.

## Problems

I tries to write a regression test, and even though I use `test_after_commit` gem, the test didn't fail. So I removed it.
@marianitadn, is this gem needed as you remove `after_commit :ensure_default_role` and changed it to `after_save`?