Different icons for HTML5 video

Resolves #1522.
Handles gfycat, giphy, and gifyoutube.
Built and tested in Chrome, no obviously apparent bugs.

I do have a few concerns about how I did this, see the comments on the diff.

Also - what do you think about the HTML5 expando icon?
okay, I apologize in advance, I have lots of comments :-)

I think we should pick a name other than "html5" - that's generic, and this isn't a new icon for all html5 videos, right?  Just for "silent videos" aka "video-ized gifs"?  There are still direct links to mp4 files and other content that serves up html5 video (medaicru.sh, for example) that have sound, and we don't really need those to have a separate icon - video is video.

Maybe just .video-silent?