Cache associated objects used in filter result pages

We cache on a per-item basis for 30 minutes. The total space used is of
the order of 500-1000KB.

I've configured the in-memory cache for production which is a
per-process cache store.
Cache organisation options used in filter menus

I removed the all_organisations_with(X) method. The feature of hiding
orgs who don't have the specific document type selected is fairly
useless as they don't get updated dynamically as you choose from the
menu.
Merge pull request #794 from alphagov/speedup-filter-page-filter-options

Related to https://www.pivotaltracker.com/story/show/55381888

Speedup filter page filter options