Create a bounding box for regions

[indiragp]

Automatically calculate a bounding box for a region based on the watersheds that are contained in it.

[selimnairb]

After discussion with @jonglee1 I'm going to make this a simple polygon field that we will (for now) manually enter via the admin console. This way we can easily set the default zoom to extents larger than the watersheds we happen to have models for at any point in time.