Check for existence of related posts target element before JS runs

Right now we assume the related posts JS file is only loaded on pages where we want to attempt to query for related post content. Some people may want to concat the related post JS file along with their other scripts for better cache-ability. We should add some checking to allow for this use case.

Start by locating the related posts JS file and the target element it queries. Check how the script behaves when that element is absent, then verify that loading it on pages without related-post content no longer causes problems while existing related-post behavior remains unchanged.

Written by the indexing model from the issue text.