Add management command to check for multiple parent types

Adding a way to check which domains have multiple options set so we can see how much needs to be fixed before merging the commit to actually prevent this.
i'm torn since it's a one off management command, but this is just screaming to use the reduce view of `locations/by_type`. it will be the difference of loading every location in the system from the database, versus making a single query per type and getting a single number back which is the exact number you need. i'll go ahead and merge this, but it would probably be a good exercise for you walk through that use case if you think you'd learn something (and like the other comments, this will run much faster if you make that change)