[Eleventy] Resurrect pagination arrows

[emersonthis]

PR #206 we switches the site from Jekyll to 11ty...

The pagination component is a bit simplified from the original Jekyll version, which had arrows that appeared conditionally depending on the current page. This is explained in a code comment.

This is definitely doable, but 11ty and Jekyll handle pagination differently, and the original template logic in the Jekyll version is a bit of a tangle.