Sanitize document filter params

This adds more thorough whitelisting and cleaning of document filter params to reduce the number of exceptions raised by script kidies sending requests with strange parameters. It uses strong parameters to filter out unwanted and incorrectly specified document filter params and encapsulates everything inside a new class: `Whitehall:DocumentFilter::CleanedParams`. 

Tracker: https://www.pivotaltracker.com/story/show/67544398