Adds simple UI for locking map position to a specific metro

This adds a simple selector for restricting the map to a particular metro. The selector is only present when an initial MSA is present (right now this means the user got to the map by selecting a specific city on a previous page). If the user scrolls outside of this range, the map gently kicks them back (like a rubber band).

The next step for this, I think, would be to dim everything outside the selected MSA.
Can we store the result of ```$('#enforce-bounds-selector')``` instead of running the query three times? Just a ```var $thing = $('#enforce-bounds-selector')``` would do.