Add image-compare widget

[ickk]

Adds a side-by-side image-compare widget for usage in news posts.

Usage in a document should look as follows

<main>
  <div class="image-compare" style="aspect-ratio: 16 / 9" data-title-a="Apples" data-title-b="Oranges">
    <img class="image-a" alt="Apples" src="apples.png" />
    <img class="image-b" alt="Oranges" src="oranges.png" />
  </div>
</main>
<script type="module">
  import { enable_image_compare } from '/components.js';
  document.addEventListener("DOMContentLoaded", function() {
    enable_image_compare();
  });
</script>

The aspect-ratio should be set to match the aspect ratio of the images, but it will default to 16/9.

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I tested this on Windows with Firefox, Chrome, & Edge. Additional testing for Safari & others would be appreciated.

[ickk]

It wasn't exactly clear in which file I should include the js.. so I made a new components.js file. I think it probably makes sense to rationalise our js utilities into only 1 or 2 different places.