Skinr rules settings fieldset for region editing needs work

The Skinr Rule settings fieldset contains all enabled themes, whether the region definition exists for that theme or not. This should not happen. Showing these options for any theme that does not have the specified region is pointless, confusing and just adds unnecessary clutter to the UI.

I think we should be checking to see whether the region actually exists based on it's system name, and only print that fieldset if it's a region that is shared across themes. For example, if someone chooses the region type: region__triptych_last, that region only exists in Bartik, and therefore only the Bartik options should appear. If there are 3 themes enabled, and all themes share a region, like region__content, then all 3 themes should appear.