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.
1. Valid point.
2. I did, regrettably, write this.
3. Renamed batch -> send_in_batches at 5183e52
4. `:-(      ----->      :-)`