Fix email notification system

[metasoarous]

At some point, we turned off the email notification system because it was getting flagged as spam (and was admittedly pretty annoying since it sent out an email on every single new comment).

The new email notification system should only message once per day, only if there are new comments, and potentially only if there are new comments which appear to be interesting (high priority value in comment routing, or based on some sense of how much we know about the placement of the user, or whether we need them to weigh in on consensus comments; see #211 for context).

Note that this is required before #114 will be able to go into effect.

[willcohen]

As a stopgap, given that there's a lot of other pieces that need to happen to finish this item, could a note could be placed on the email sign up card clarifying that emails are currently disabled? This is the biggest friction point I'm currently observing when trying to run a conversation, that people are getting frustrated when they find that the emails aren't being sent as promised.