Export list of editions according to current filters

Provide functionality to export the currently-selected list of documents as a CSV from the admin editions index. This is implemented as a Sidekiq job which emails the completed list to the requesting user. Additional fields are included in each row, this list of fields is defined in the presenter and will probably change in the future.

https://www.agileplannerapp.com/boards/173808/cards/6307
I also like the idea of a separate method for unpaginated results. It also makes it less likely that we'll get DDOS'd by someone passing the `unpaginated` parameter to the `index` action.