Drag to Merge Journals

This commit shows were and how journals can be merged. This is a work in progress that will require some experience with real world journals and editing workflows. For now we get to see the merged pages but they are not saved back to our sites.

Intended workflow:

* notice that a page you have forked and edited has a newer version.
* open the newer page side-by-side with your fork
* drag the journal of the source page to your forked journal
* see a merged page appear as a 'ghost' to be reviewed and then (eventually) forked into your own site.

This work comes from a collaboration at the 2014 Indie Web Camp, Portland, Oregon.
http://indiewebcamp.com/