Add auto update javascript

Make search results auto update when facets are changed on the finder.

This pull request does the following:

- Lifts the js from /alphagov/frontend/live_search and rewrites it using prototypes to match the style of the other js in this project.
- Modifies the js to work with this DOM and mustache config + updates tests.
This gets set to a value on line 14 below, no methods are called in between here and there so this is probably redundant.