Merge changesets.CreatedByCampaign and changesets.OwnedByCampaign fields

Created by: mrnugget

As a follow-up to #12435, which introduced the OwnedByCampaign: bigint field, we can probably get rid of the CreatedByCampaign: bool field and merge those two.

The latter is used for analytics purposes and is set once the a changeset has been created on the code host.

So: changeset.CreatedByCampaign == (changeset.OwnedByCampaignID != 0 && changeset.PublicationState.Published().