Send email to user if message has not been read for 3 minutes.

# Problem

From https://github.com/code-corps/code-corps-api/issues/1234#issuecomment-349187243

The idea behind the messaging feature is that it ought to work mostly in realtime, while still supporting async communication.

In order to achieve that, we need to be able to send an email to the user if the message sent to them has not been marked as read after 3 minutes.