Fix for #1622: Validate that facility name is unique on the sharing network (as best we can)

Prevent `Facility` objects with a duplicate name, within a sharing network.

Note that we cannot guarantee uniqueness (since other data may not have been synced in), and we shouldn't force uniqueness (because of the above).

Testing:
* Tested that this works locally.
* Needs to be tested that this doesn't break syncing if syncing a `Facility` with the same name.