Feature/new datamodel

A more or less complete draft of a data model for merging datasets into exhibits.  Additionally, property definitions have been split out into models.  References to the dataset are still intact, as data source information remains to be migrated.  One significant change is that there are now two exhibit data models.  'PublishedExhibit' is a saved exhibit that is visible to users.  'DraftExhibit' is what is edited.  When the editor opens, a draft is created if it doesn't exist.  When the user clicks 'Save', the draft is copied into the published exhibit and discarded.  If a published exhibit doesn't already exist, the user will be presented with a dialog and given the opportunity to give it a title, etc.  

The following items should be completed before this is merged:
- [ ] Build out the data views for display
- [ ] Build out views for data property editing, and add a transactional pipeline for augmentation
- [ ] Modify the data ingest pipeline to create a DraftExhibit on load
- [ ] Migrate the DataSource infrastructure to point to exhibits, and add a ReferenceDataSource for datasets with more than one exhibit