R1:a894c9993512

Add "max-width: 100%;" to stop large images from overflowing the new rendering engine UI

Summary:
Fixes T13148. Ref T13105. The new document rendering engine for images let them overflow the UI bounds.

Add `max-width: 100%;` to keep them contained.

Test Plan:
- Viewed a very wide image in Safari, Firefox and Chrome. Saw sensible rendering.
- Also viewed a normal image, saw normal behavior.

Reviewers: amckinley, avivey

Reviewed By: avivey

Maniphest Tasks: T13148, T13105

Differential Revision…