Confirmation email still sent for allow-listed users

[lambdaTotoro]

I just did a test for an unrelated feature locally and found out that currently, if self-serve authorization is enabled, the email is always sent out, even if the user that's signing up is already listed as an admin.

There's no big harm done, because the only thing that could happen is that anyone clicks on the link to find an 'already authorized' message, but that really shouldn't be the case. I wonder if we could have a direct 'thanks for signing up, you were pre-approved, you can now log in'-type message instead.