Set "max-width: 100%" on "size=full" Remarkup images

Summary: Fixes T5500. When an image is embedded with `{Fxx, size=full}`, add "max-width: 100%;" so that large images are scaled down to the size of the container. This seems like a better and more reasonable behavior than having them scroll. You can still lightbox them or right-click -> view if you really want the full image.

Test Plan: Dragged window around with a very large `size=full` image. At large window sizes, the image displayed at 100%. At smaller window sizes, the image was scaled to fit.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5500

Differential Revision: https://secure.phabricator.com/D9758