Add reminder for upcoming scheduled pairing

[jdkaplan]

One feature I like from the (coffee) chat bot is that it gives me a reminder/warning that I'm scheduled for matching before it actually makes the matches. It gives me a bit of time to issue a skip command if I decide that I don't want to get matched the next day.

This is a good feature for Pairing Bot to emulate! This could be another job that runs a few hours before the real pairing job that messages each person scheduled for the upcoming run with a reminder that it's happening soon (and how to skip?).

[blvrd]

I can take a look at this one.