Add a "Session confirmation deadline" date to Event terms

Problem/Motivation

Currently event organizers can bulk moderate sessions, but the follow-up is manual.

Proposed resolution

In the Event Details config page, add a "Session confirmation deadline". If this date is within a set period of time (e.g. one week), send a reminder email to the authors of any accepted (but not confirmed) sessions. After the date has passed, automatically move them to declined and email the site administrator(s). (The latter two pieces have been moved to child issues)