Improve experience around imgur albums slightly

Made several small changes to address the direct issue in #954 - where if you have an /a/ link it tried to display but broke in the proxy.  Altered to avoid proxy, to allow for a wider range of pages, to skip already-image links.

Note that this doesn't entirely address the issues with imgur - we still don't know when a link is an image's page or an album's page if we are under /gallery/ (maybe need to bring in imgur API?), and there's no way to separate eg "/random" from valid images (other than building a big whitelist).