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.
Oh! Thanks. In the git diff view it wasn't at all clear that those were inner functions of the do_forums_digests_flagged. Now I think I'm squared away.