OperatorsInFeed: Should be deleted when Operator is deleted

[irees]

When an Operator is deleted, it should also clean up any OperatorInFeed records that reference it.

Example: an Operator was removed by a changeset. https://github.com/transitland/transitland/issues/113

[irees]

Manual cleanup of the following OIFs:

irb(main):004:0> oifs = OperatorInFeed.all.select { |oif| oif.operator == nil }
irb(main):005:0> puts oifs.to_json
[{"id":15,"gtfs_agency_id":null,"version":1,"created_at":"2016-02-10T20:26:33.547Z","updated_at":"2016-02-10T20:26:33.547Z","operator_id":10,"feed_id":15,"created_or_updated_in_changeset_id":518},{"id":16,"gtfs_agency_id":null,"version":1,"created_at":"2016-02-10T20:28:42.067Z","updated_at":"2016-02-10T20:28:42.067Z","operator_id":11,"feed_id":16,"created_or_updated_in_changeset_id":519},{"id":394,"gtfs_agency_id":"ACTV S.p.a","version":1,"created_at":"2016-07-15T02:21:18.266Z","updated_at":"2016-07-15T02:21:18.266Z","operator_id":367,"feed_id":317,"created_or_updated_in_changeset_id":1264},{"id":450,"gtfs_agency_id":"RTL","version":2,"created_at":"2016-07-29T18:27:40.352Z","updated_at":"2016-08-22T16:01:31.316Z","operator_id":422,"feed_id":58,"created_or_updated_in_changeset_id":1737},{"id":624,"gtfs_agency_id":null,"version":1,"created_at":"2016-08-25T18:32:26.567Z","updated_at":"2016-08-25T18:32:26.567Z","operator_id":601,"feed_id":404,"created_or_updated_in_changeset_id":1739}]
irb(main):007:0> oifs.each { |oif| oif.delete }

The operator_id is not nil, but is a dangling reference to the Operator that was deleted.

[drewda]

@kkowalsky followed up with support requester under ticket #999