Add a unique constraint between building id and bgroup_id
Created by: cschloer
https://blocpower.myjetbrains.com/youtrack/issue/DASHB-412
This was previously handled with the front end restricting adding buildings that had already been added. Now that we are automating some adding of buildings to BGroup we should add this constraint.
Here's the migration:
ALTER TABLE building_bgroup ADD CONSTRAINT bid_bgid_unique UNIQUE(building_id, bgroup_id);