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.
`do_forums_digests_flagged` actually only makes 2 function calls, after defining a few helper functions:
```
    messages = get_messages(input_file)
    batch(messages, is_dry_run)

```