Add support for storing documents as 1st class items

[pmackay]

@GilesGibson if we have a go at this, would it make sense if it was done such that "Document" was a content type the same as "Visualization" is? That looked like a key request from both Mark and Liz, but want to make sure its done the right way.

[GilesGibson]

It would be good to have a document as a content type. If it could be viewed etc as a pdf but in the visualisation window (scrolling) that may work. It could then be added to the dashboard. It could also have comments round it when stored as a visualisation.

[pmackay]

Would you want the documents to be mixed in with the visualizations on the main list, or a totally separate list?

[GilesGibson]

Hmm, not sure, probably a separate list. Visualisations are something created on the site, a different approach. Docs/pdfs etc are imported and static.

[pmackay]

First cut of document storage:

http://live-communitydata.gotpantheon.com/documents
and
http://live-communitydata.gotpantheon.com/lambeth-borough-report-2014