Feature/editor property items

This pull request fixes record navigation in the property editor. The property editor used to assume that every ExhibitProperty's PropertyData would contain the same number of data values in PropertyData.json. This isn't the case.

I added code to create a list of PropertyData.json **id** values. This acts as our record count for navigation. It also lets us click forward and backward through the records. Something feels wrong about how I generate the list of ids in viewshare/apps/exhibit/static/dataset/js/models/editor/property-collection.js. There's a 3x for-loop but it works. I'm open to suggestions for improvement.