This splits out all the URL computation logic from $.fn.retinaGravatar()

into a new Djblets.getGravatarForDisplay(). This can be used in cases

where the HTML for a piece of UI is being generated and isn't yet part

of the DOM, and can be more efficient than post-processing existing

elements.

Tested that existing Gravatars were continuing to work.
Used this function in some code that was building HTML fragments. It

resulted in Retina-friendly Gravatars.