Fix image sizes in Markdown content.
Review Request #10542 — Created April 25, 2019 and submitted
Overly-large images would overflow the rendered Markdown field, and this
was easy to have happen when dropping in high-DPI screenshots into a
text field. We now cap the width to the container width, preserving the
aspect ratio, so that it appears correct.
Tested with some large images. Saw that they no longer overflowed.