Wait until Commons CentralAuth image is loaded

This is a workaround to help get rid of "You must be logged in to
upload" errors in photo uploads. It works in the following way:

* CSS hides the "Return to" link and shows a spinner instead.
* When DOM is loaded, JS checks if a Commons CentralAuth image is present
  on the page. If not, it show the "Return to" link.
* If it is, it waits for the image to load, and then shows the "Return to"
  link.
* If there are errors when loading the Commons CentralAuth image, we log
  them using EventLogging.

Bug: 45452
Change-Id: I02858fdf2745c3dd5c9be4ee832b322ff20cc640