support "back" button in the browser for map changes

[wbazant]

Reproduction: be at any URL, for example http://beta.fallingfruit.org/map/@49.190439178461446,-122.93233531363494,18z . Scroll in - then the URL in the address bar changes, and map is refreshed. Press back button -

• in Firefox the URL changes back, but the map is not refreshed
• in Chrome the browser transitions away from beta.fallingfruit.org entirely

[wbazant]

Similar bucket as #359, but about the browser back button and not the UI element in open location