Allow users to verify via slash command their email to gain access to the server

[weakphish]

Users, when prompting for verification, will be asked their email. If the email domain is whitelisted, they'll be sent a verification code. Once the correct code is supplied, they will be added to the verified role.

Admins should be able to add emails to the whitelist

This can be accomplished using an ephemeral slash command with an email as an argument in order to hide the user's emails and encapsulate them to the given server.