Re #177: Create AR model for Europeana records.
Re #177: Implement harvesting of WWI related Europeana portal content.
Re #166: Remove Sphinx-related deploy hooks.
Re #177: Only harvest Europeana records not already present.
Re #177: Proof of concept for merged search results.
Re #177: Continue harvest on Europeana API error "Invalid record identifier: ..."
Re #177: Require record_id and object Hash on EuropeanaRecord model.
Re #177: UI fixes for merged UGC/institutional "collection" controller.
Re #177: In merged search results, present "Source" radio button; only show facets when single source selected.
Re #177: Fix data provider facet label for Europeana portal results in merged display.