Block temp emails from signing up for the Slack and email them #311
Description
To prevent spam and account abuse, we should block temp emails from signing up for the Slack.
This should be fairly easy to do using a package like mailchecker when Toriel sends out an invite.
If the email is a temp email, Toriel should not invite the user to the Slack, and there should be an email sent to them using a transactional loops email letting them know that temp emails are not allowed to signup for the Slack, and that they can reach out to [email protected] if they have any questions.