Fix Issue 1922 - Allow image resizing when zooming

Following up from @brunodea's initial work on this in https://github.com/ankidroid/Anki-Android/pull/300. Use javascript to constrain image dimensions or use CSS if we're on the newer Chrome engine.

I've opted to use 95% for the max size since that's what the [desktop client uses as well](https://github.com/dae/anki/blob/master/aqt/reviewer.py#L333).