Handle "invites" when someone is already in the slack

[jaspermayone]

Recently we had a user complain they weren't receiving invites to the slack. Turns out they had already received one, and so it doesn't send a new one.

We should find a way for Toriel to alert a user (or mb it's on the hackclub.com/site page) when this is the case, to eliminate confusion.

CC: @tregsthedev who caught this!

[radioblahaj]

We should have it send a email if someone is already in the database or alert us somewhere in #welcome-committee maybe?

Message ID: @.***>

[jaspermayone]

I think it should email them and CC slack@ or one of us

[jaspermayone]

Welcome Committee should only be for welcomes

[jaspermayone]

if we want it on slack we can add an additional channel

[radioblahaj]

Closing this until Toriel is being used again