Add dry-run option to flagged forum post digest

A new "--dry-run" flag has been added, which will print the course_id
and email address for intended recipients, instead of sending an actual
email.

Other, minor changes have been included, in preparation of deploying
this feature.
I'm fuzzy on how is_dry_run makes it from do_forums_digests_flagged() to batch(). Could you connect the dots for me?