Remove Slack from the website, replace with Discord

> "I went and removed slack from most places on the website.
> We'll be moving to discord 100% when the space reopens
> The board has already voted to get rid of slack
> We've just got to get a good moving process setup
> I think I've done that on most places on the website
> If I've missed any, let me know or make a pr"

 - @miketweaver 