feat(email): allow email addresses to be opted out for build notifications

We still need a way for email addresses to be marked as opted out. Things needed for that:

- [ ] Determine where to receive opt out requests (probably in API, possibly with a frontend in -web?).
- [ ] Add a link to that place in the email footer.

This is a very simple implementation. I chose Redis as a place to store this, since I think Postgres isn't a very good choice for this (it's just a set that we need to add emails to and check if an email is a member). Redis has the set operations we need and [according to the documentation](http://redis.io/topics/data-types#sets) it can store up to 2^32 - 1 members, so I think we should be set for a while.