This fixes an important bug in doc generation where Retina filenames

weren't being generated when there were image files with the same name

in the tree. When Sphinx has two images in a tree sharing the same

filename (or one directory references another's image), one of the

images get renamed to include a numeric suffix. The reason is that all

images end up in the same directory and are not namespaced in any other

way. When this happened, we'd fail to create an equivalent Retina image,

breaking support on one or more of the pages.
This also adds a configuration option for Retina suffixes like @2x

and @3x. These are the default suffixes that will be tried, but

additional ones can be added on conf.py.

Built docs for Review Board and saw that the Retina image issues we've

been encountering for one of the images has gone away.