Infinite scroll for job listings

Suggested here: https://wordpress.org/support/topic/endless-content-instead-of-load-more-pagination/

I imagine we could have it be something like, use the "Listings Per Page" setting but have an extra "Infinite scroll" checkbox option.

Review the linked WordPress support proposal, then trace how the “Listings Per Page” setting and current job-listing pagination work. Define the checkbox behavior and verify that additional listings load continuously instead of using load-more pagination; the issue names no files or tests to run.

Written by the indexing model from the issue text.