Rework association between series and documents

Instead of linking series with editions, we now link directly to the
document. This removes the need to copy across the association with
each new edition and greatly simplifies the code in general. We no
longerr list series documents grouped by type on frontend - this added
little value to the user.
DRY things up: kill off DocumentSeriesFinder

SluggedModelFinder does exactly the same work
Rework Importer to work with new doc series assoc

I'm not entirely happy with the way more code is being chucked into
Import#perform, but without a wholesale refactor of the way the import
engine works, I can't see how else to handle this.
Remove fields for doc series from editon form
Admin::EditionFilter page size can be overridden
Introduce tabbed management for document series

Document series documents are now directly managed through the
document series, and are no longer added/removed from the documents
themselves. A search interface allows for documents to be found and
added and removed from the series. It was intended that this would
be an AJAX-powered autocompleting search, but the response time for
searches is just not fast enough without a full-text search engine
behind the scenes. Currently, there is no fallback for non-JS browsers
Include document count in series tabbed nav
Add some visual styling to doc series
Update doc count and remove unused templates

There is no index action on the document series controller - the
organisations controller has a document_series action instead. Leaving
reworking this to be more RESTful for the story that will untie series
from organisations.
Improve document series adding feedback

Search results that are already part of the series will be identified
as such. We also update the search results to reflect once a document
has been added. To do this, I've moved the search action out of the
main editions controller. I had hoped to make this a generic search
action, but the mustache template was already tied to document series
management, so it was only a half measure anyway.
Add warning message regarding document series
Search result visual tweaks
Handle non-published editions in search results
Update DOM ids in mustache template
Rework document series management scenarios

Regressions for the key features and those not covered by existing
functional tests
Rename partial to reflect better what it is.
Invoke mustache compilation for cucumber features

We now have cucumber scenarios testing admin features that rely on
mustache templates. To ensure these work, we need to compile these
templates before the features are run.