[ally.is.focusRelevant] clean up <embed> mess

[rodneyrehm]

By now we know that properly supporting the <embed> element means to understand every file type in every browser - as video/mp4 is treated differently than video/ogv than image/svg+xml, than $insertRandomMimeType.

ally 1.0.x made an ill-advised effort to accommodate the different types. After examining the current tests and running a few more I have to admit that ally isn't doing a good job with this element and there is no easy fix.

I'd like to demote <embed> to the following fixed state, regardless of content type:

{
  focusRelevant: true,
  focusable: false,
  tabbable: false,
  onlyTabbable: false,
}

By considering <embed> focus-relevant we're making sure that ally.maintain.disabled (by way of ally.query.focusable({ strategy: "all" })) can still disabled any <embed>, whilst any other strategy ("quick" and "strict") will not return any <embed> elements anymore.

Authors should be embedding SVGs inline or via <object>. SWF is embedded via <object>. Videos and audio files should be embedded via <video> and <audio>. That leaves <embed> for obscure plugins that we wouldn't understand anyway.

@marcysutton any objections to "killing <embed>"?