Add individual snippets reminders for slack/hipchat

Right now we send a personalized email to a person who hasn't written snippets, saying "They are due tomorrow!" But for chat, we only send a generic message. It would be nice to support a 1-on-1 message from snippet-bot to those users who still need to submit snippets. (It could also be on sunday night, or we could do monday a bit before snippets are due.)

Implementation-wise, I think what we would do is to look at app-settings to see what chat services are enabled, and for each such service put a field on the user-settings which is something like " username" or other id. Then we'll use that to send the 1-on-1 message.

Start by locating the app-settings and user-settings handling, then trace how the existing personalized email reminder is scheduled and sent. Check how enabled chat services and user chat identifiers are represented. Done means users who still need to submit snippets receive individual reminders through each enabled chat service, with the delivery timing resolved.

Written by the indexing model from the issue text.