[fix bug 1331277] Always set window.ga
[fix bug 1331277] Always set window.ga

An adblock filter could possibly block downloading of ga.js and thus window.ga
will remain undefined. This brakes listings filtering and a couple of other
functions.

This patch checks if window.ga is defined and if not defines it as a noop
function.

alexgibson/lumbergh,glogiotatidis/lumbergh,glogiotatidis/lumbergh,mozilla/lumbergh,mozilla/lumbergh,glogiotatidis/lumbergh,alexgibson/lumbergh,mozilla/lumbergh,mozilla/lumbergh,alexgibson/lumbergh,glogiotatidis/lumbergh,alexgibson/lumbergh