use vignette for generating avatar urls

@drsnyder 
Use `VignetteRequest::fromConfigMap` to generate vignette urls for user avatars. We can't use `fromFile` because avatars aren't stored as file objects, they're strings built from the user's id.

Also piggybacking - use `$wgUploadPath` for determining a file's "prefix path" instead of `$wgContLang`. This is needed because some english wikis have their images in `<bucket>/en/images` instead of `<bucket>/images`, and also some wikis have their bucket stored under another, higher level bucket (ex: `p__/psychusa`), so we need to parse that to determine what `parse-prefix` to set for the vignette url.
We should test this stuff. These are the kinds of things that our future selves will forget about and having tests will also serve as documentation.