Improved ad block detection

[davidfischer]

This improves our ad block detection to avoid any false positives that would show the ad block nag.

When the ad fails to load, we add an element which we can detect whether it is visible.

Fixes #4402.

[humitos]

The logic sounds good.

Since we are adding this element to the body just to test that it's not blocked, don't we need to remove it once we know that it's not blocked?

Also, I suppose that our class ethical-rtd will create an element always with a height > 0 even if it has only one char ( ).

[davidfischer]

don't we need to remove it once we know that it's not blocked?

I suppose we could remove it regardless.

I suppose that our class ethical-rtd will create an element always with a height > 0 even if it has only one char ( ).

Correct.

[humitos]

Feel free to merge.

[davidfischer]

Feel free to merge.

I'll make the changes. I also noticed that jquery doesn't correctly report status codes for JSONP. Cool...