Previously video embeds were given a fixed with of 369px, which is close
to the width available to annotation content at the sidebar's default width
(~380px). On narrower viewports the video would get cut off.

Fix this by setting the width of video embeds to `100%` and using a CSS
hack [1] to set the height of the video such that the embed has a 16:9
aspect ratio, which is what YouTube optimizes for.

 - Set `width: 100%` on `excerpt__content` and `markdown-view` so that
   the annotation content always fills the available width.

 - Add support for specifying a class name for embed containers when
   calling `replaceLinksWithEmbeds` and use that to give embeds rendered
   in `MarkdownView` a `width` of 100%.

 - Change `replaceLinksWithEmbeds` to wrap iframes with an aspect-ratio
   box (see [1]) which causes the iframe's height to be adjusted as the
   width changes to have a 16:9 aspect ratio.

   Adding this container required changes in `media-embedder-test` to
   allow the `<iframe>` to be wrapped in a container element.

[1] https://css-tricks.com/aspect-ratio-boxes/