Fix for #1882: 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 (both creation of unique names, and fail on non-unique names).
* Needs to be tested on central (that creating names on different zones works)
* Needs to be tested that this doesn't break syncing if syncing a `Facility` with the same name.

![image](https://cloud.githubusercontent.com/assets/4072455/2689741/229c2cba-c31b-11e3-9209-7cfaed4a6634.png)