Split out Organisation#show into #documents

This should speed up the organisation pages dramatically, as they no longer load documents on #show.

Additionally we're now paginating the list of organisation editions, to make the featuring experience faster. I've kept the currently featured items at the top of each page to allow for easy unfeaturing, but they're excluded from the "all editions" list to prevent duplicates.

The navigation between tabs should remain the same, as I've co-opted the tab interface to provide a proper URL instead of a fragment identifier.

https://www.pivotaltracker.com/story/show/44486835