Migrations and OperationalImporter updates #3615

This makes `being_followed` attribute on `Participant` true by default.  Only a minor adjustment in the `OperationalImporter` was needed to `set_participant_being_followed_by_ids` and no change to `set_participant_being_followed_using_heuristic` is necessary; the 'old' logic should still hold when trying to heuristically determine which participant is being followed.