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?
I wondered that too, since "collapsed" is already added on line 680, and it still seems to work fine.

I'll make those class changes you suggested, ignore my commit from 2 minutes ago.