Feature/view preview

This adds side by side preview for exhibit view construction.  It also separates the generation and editing of lenses into a separate editor.  The next steps are as follows:

- [ ] streamline the process by allowing the editing of lenses in their respective views.  The global lens editor is a sort of expert mode.  
- [ ] make the default view of views in the builder be the preview, with an edit button.  Clicking the button will open the appropriate side by side editor, exactly like the current facet workflow 

This pull request is basically just a functional checkpoint, so other work can continue in parallel.