Don't drop "phabricator-remarkup-embed-image" class from Remarkup images with width or height

Summary: Ref T10110. If an image had width or height, we would accidentally not give it an $image_class.

Test Plan:
{F1057988}

{F1057989}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10110

Differential Revision: https://secure.phabricator.com/D14983