This PR updates the logic that executes a new sponsorship contract to query for existing and active applications from that same sponsor and "deactivate" them.

The sponsorship model now has a new FK called overlapped_by to flag these existing applications as overlapped by the new one. Beyond enabling us to understand which application replaces another, this FK is also being used to control which sponsorships should be considered as active ones. If this FK is populated, the application gets excluded from the Sponsorship.objects.enabled() queryset, even though it is finalized and has a valid start/end date range.