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.
```python
def do_forums_digests_flagged(input_file, is_dry_run):
    # ...
    batch(messages, is_dry_run)

    def batch(messages, is_dry_run):
        # ...
        send(batch, is_dry_run)

    def send(batch, is_dry_run):
        if is_dry_run:
            for message in batch:
                email = message.get('recipient', {}).get('email')
                course_id = message.get('course_id')
                print("{course_id}	{email}".format(course_id=course_id, email=email))
        else:
            generate_and_send_digests_flagged.delay(batch)

```