Allow width/height attributes on <source> tags.

[buckett]

Allow width/height on <source> tags so that when contained width a <picture> tag the browser can reserve the correct amount of space and not re-layout the page once the image is loaded.

Test plan:

• Create a wiki page and put in tags. e.g.

  <picture>
  <source srcset="https://interactive-examples.mdn.mozilla.net/media/cc0-images/surfer-240-200.jpg" width="240" height="200" media="(max-width: 600px)"/>
  <img src="https://interactive-examples.mdn.mozilla.net/media/cc0-images/painted-hand-298-332.jpg" width="298" height="332" alt="" />
  </picture>

• Check that after saving the height/width are persisted in the page.

closes gh-2272

[buckett]

This his been fixed through a different commit and it no longer needed.