Ensure `page_embed.*` work sensibly without the enhancer

[reefdog]

The enhancer is our personal entry into the embeds, but people should be able to load and use the page embed libraries directly without using the enhancer (or by using their own enhancer). Make sure there are no assumptions within page_embed.* that require the enhancer.

[reefdog]

Our direct-load example on the sample page works:

<div id="foo"></div>
<script src="https://assets.documentcloud.org/embed/page/page_embed.js"></script>
<script>
  DocumentCloud.embed.load("https://www.documentcloud.org/documents/282753-lefler-thesis.html#document/p57", {
    container: '#foo'
  });
</script>

To get styles, you'll also need the CSS: <link rel="stylesheet" href="https://assets.documentcloud.org/embed/page/page_embed.css">

Not marking this as complete because I want to really test the bounds of this ability, but atm it does work if implementors want to play with it.