invites: Remove invites worker, make confirmation object in-process.

The "invites" worker exists to do two things -- make a Confirmation
object, and send the outgoing email.  Making the Confirmation object
in a background process from where the PreregistrationUser is created
temporarily leaves the PreregistrationUser in invalid state, and
results in 500's, and the user not immediately seeing the sent
invitation.  That the "invites" worker also wants to create the
Confirmation object means that "resending" an invite invalidates the
URL in the previous email, which can be confusing to the user.

Moving the Confirmation creation to the same transaction solves both
of these issues, and leaves the "invites" worker with nothing to do
but send the email; as such, we remove it entirely, and use the
existing "email_senders" worker to send the invites.  The volume of
invites is small enough that this will not affect other uses of that
worker.

Fixes: #21306
Fixes: #24275

scripts/lib/check_rabbitmq_queue.py

@@ -15,7 +15,6 @@ normal_queues = [
     "email_senders",
     "embed_links",
     "embedded_bots",
-    "invites",
     "missedmessage_emails",
     "missedmessage_mobile_notifications",
     "outgoing_webhooks",