Once we email someone an invite, honor the invitation

Previously, we were waiting to decrement the invite_count until a user
confirmed their email address, which created weird situations where we would
email people only to have them get a 500 when trying to sign up. Now, we
decrement the invite_count upon sending the email and always honor the
invitation.

Co-Authored-By: Joseph Lyons <joseph@zed.dev>
Co-Authored-By: Max Brunsfeld <max@zed.dev>