Feature/data source list

This branch adds a list of pending data sources (ie, data sources without associated saved data sets).  It also hides the data loading transaction mechanism, ensuring there can only be one 'open' transaction at a time.  To this end, a 'complete flag was added to the transaction model.  Finally, the 'Inspect' page for data sets lists the data source information (visible to those with edit permissions on the data source).

A few 'nice to have' things outstanding:

* Integrate the transaction status into the data source description, and restore the auto-update functionality for running transactions
* Workflow improvements (descriptive text, etc.)
* Data source delete (distinct from dataset delete)
* Data source 'Inspect page' including transaction history -- there is a page, but I left it unlinked, as it's currently just the 

Considering that we're planning on collapsing this all into the Exhibit app, I'm not sure how far we want to go with the UI, as it'll be radically simplified shortly.