Expose flagged property in Reportback response

[weerd]

Current Behavior

Prior to filtering reportbacks in the response for the /reportbacks endpoint, we would only show certain properties why_participated and flagged if the requester had access to view those items. However, now that we do some filtering ahead of time and don't show RBs that should only be visible to admin users, we should be able to expose the flagged property.

Desired Behavior

Show the value for the flagged property in the reportbacks endpoint response.

Why This Matters

This will also fix a slight bug with how the flagged property would be expected but not always available if it was an anonymous request.