Build infrastructure for per-page JavaScript loading and use it for filters.

Check the "data-page" attribute of body with $("body").data("page") to
see what controller and action the page is being redered for. The data
will contain a string of the form "Controller#action", i.e.
"Bounties#index" for the main page.  Your JavaScript can check for the
controller or index it matches before executing any logic if it should
only be run on certain pages.
Combine all scripts handling aspects of displaying the main bounty list into the filters.js.
Intermediate commit.