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?
Unfortunately we'll have to use something like:

    a.toggleImage.expando-button.video.video-muted

to override reddit's spritesheet which uses:

    .expando-button.video.expanded:hover

and we'll have to use something similarly specific to style .commentImg:

    a.expando-button.video.video-muted.commentImg