This feature will be used to send a daily digest email with a list of
flagged forum posts to the courses' moderators.

This exposes a new management command: `forums_digest_flagged`.

The command can optionally be configured to run at regular intervals by
setting FORUM_DIGEST_TASK_INTERVAL_FLAGGED to be >= 1 (minutes)
(default=0/disabled). Setting this value to 1440 will run the command
daily at midnight. If enabled, this command is scheduled alongside the
`forums_digest` command via `./manage.py scheduler`.

This command is intended to pull the list of threads via the Heroku
toolbelt, which must be installed. This command can be updated by
changing the value of COMMAND_FETCH_FLAGGED_FORUM_POSTS.

Note: Moderators cannot currently unsubscribe from this digest.