Topic/historyjs

This adds back button functionality support using History.js

When a user pans/zooms on the map, the center position and zoom level is added to the url. When a user browses to a different page and hits the back button, the url with the last position and filters they were looking at gets loaded into the page.

As an aside, this also enables more easily sharing a map view as well since the url actually means something as opposed to staying unchanged.