PLA-415 -- add varnish caching to search results pages

This changeset adds in varnish caching of 12 hours to anonymous searches. 

This also includes a new feature to allow special pages to set varnish caching, which was not previously possible according to our controller model for special pages, and how it interfaces with core MediaWiki.

This solution was worked on in cooperation with @owend and @Wyvek.