New document series interface

This is partial delivery of the story: https://www.pivotaltracker.com/story/show/54769324 - basically, everything but the actual ordering of documents within a series.

It reworks admin such that documents in a series are directly managed from the series itself, rather than adding/removing them on the edition editing page. The new interface has a document search form that allows documents to be found and assigned to the series. I've not taken the time to make this fallback for non-JS users as we want to get something out for the end of this sprint. 

Document series are now displayed slightly differently on the front-end website as well - instead of being listed grouped by their document types, they are now listed in a single list in reverse chronological order.

It contains a data migration to migrate the association with series from editions to documents. The code has been written such that the front-end code will still display the correct documents within a series before the data migration has been run. However, the new admin interface will not list the correct documents for series until it has been run. Once the data migration has been run, the shimmed methods and associations in the DocumentSeries model can be removed.

There are some significant JS additions here (i.e. the introduction of mustache into admin), so @bfirsh might want to take a look to make sure everything is cushty.