Per-category email preferences

Email is now opt-in per category instead of all-or-nothing. Digests, new-match alerts, product news and launch announcements are each their own switch, so you can keep the updates you find useful and silence the ones you don't.

The goal is to make staying subscribed an easy call. Rather than unsubscribing from everything the first time one email misses, you trim it down to exactly what you want and the rest stops arriving.