Add facilities for reloading gravatar images in retina resolution.

This adds a jQuery function which, when applied to an <img class="gravatar">
will parse the URL and reload it at the appropriate resolution for
high-resolution devices. This requires some javascript on the reviewboard side
as well, to call it in the relevant places.

Used this on my local install and saw it fetch a 64x64 image instead of a 32x32
one for the gravatar in the upper right corner of the page.