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.

Edit: I went back to 90%. 95% still seemed to generate scrolling vertically (it looks like the top padding is slightly larger than the bottom padding, but I don't understand why).