Photo and Video sizes

There is a well documented issue that `img` and `video` tags without a preset width and height cause pages to repaint when the media is loaded.

In order to get a truly smooth web reader experience I think microsub servers will need to return height and width for at least the media properties (`photo`, `video` & `featured`).

Outside of that sizes for images and videos inside html content would be great too, but I imagine that would be much harder to accomplish

The issue names no files or tests; begin by reviewing the Microsub specification and its definitions of the photo, video, and featured media properties. Clarify the required dimensions for those properties and whether sizing media inside HTML content is in scope; done means the specification records an agreed, testable requirement.

Written by the indexing model from the issue text.