implement a legistar<->document cloud redirector

[JoeGermuska]

Our friends at Datamade are interested in taking advantage of the Document Cloud archive we're building. It's possible for anyone to search Document Cloud to get "our" copy of a document originally in Legistar, e.g.
https://www.documentcloud.org/public/search/account:%20%2212872-knight-lab%22%20project:%20%22Chicago%20City%20Hall%20Monitor%22%20Source:%20%22http://ord.legistar.com/Chicago/attachments/4fc6aad9-98e2-41ff-8a24-22ca5b38d49e.pdf%22

But maybe it would be easier for people (and more robust) to have a URL like
https://cityhallmonitor.knightlab.com/leg_to_dc/4fc6aad9-98e2-41ff-8a24-22ca5b38d49e.pdf which would redirect to the DC page (or otherwise make something slightly less dependent on the exact interface between CHM and DC)