Revert "Prevent image outputs from being wider than their container." - #7818

Given the various options:

1. fix aspect ratio problem in Prevent image outputs from being wider than their container. #7721
2. scroll large images within container (Disable output contents from overflowing page width. #7718)
3. (this) allow large images to overflow container (2.x behavior, more noticeable now that we have gray background)

I'm slightly inclined to keep the 2.x behavior, even though it makes it easy to produce slightly ugly output. If there were an easy way to express "don't confine to width", then I might be inclined to keep #7721, assuming a simple fix for the aspect ratio is found.

Given that #7721 as-is doesn't quite work, and causes problems when larger images are desirable, I think we should go with this PR for 3.0. We can try to get a better version of #7721 with fixed aspect ratio and that allows explicit disabling of the shrink-to-fit behavior for 3.1.

Thanks for opening this @takluyver