Added date checking to fireplace (bug 1044230)

That `<div>` needs to exist in app.html (which is the template used by the packaged app) and in https://github.com/mozilla/zamboni/blob/master/mkt/commonplace/templates/commonplace/index.html to work with the hosted app.

Since we don't really care about the hosted app (if the system date is wrong, the SSL cert for the homepage itself will be problematic) maybe you should instead check for the existence of the `<div>`, or introduce a setting that will only be true in the packaged app, to enable/disable the check.