Change confirmation email to be once per domain (rather than per post/follow)

[ameeker]

Currently, the user has to confirm the option to follow replies or comments every time they choose to follow a comment. I think it will be better to let them just confirm once for the site as a whole, rather than having to do it for every post/comment they want to follow. As I've been testing, I've realized that it's annoying to have to confirm it every time.

This isn't a blocker to resubmitting to .org, but we should change it before we go live.