Feature/lastharvest

[MerlinZhang]

New Feature

Keeps track of when sources were most recently harvested. A new database model called LastHarvest keeps records of each source and the most recent date when it was harvested. The database is updated every time a new document from a more recent date is normalized. Code updates include a new view and url to view the database information for most recently updated sources.

[jeffreyliu3230]

Can you add tests to this feature? We normally would require tests to be implemented for every new feature.