Show all categories in the dropdown instead of only categories with jobs

As requested in https://wordpress.org/support/topic/show-all-the-categories/

In the category dropdown, it only shows categories that have an active/unexpired job listing. We could add an option or a filter to allow folks to see all categories instead. I suppose people might want this to show the whole gamut of categories that are potentially available on the site.

This ties in nicely with #1977

Start by locating the category dropdown implementation and the query or filter that limits it to categories with active or unexpired jobs. Check the related discussion and issue #1977 for context, then make the available behavior configurable so completion can be verified by showing all categories when enabled.

Written by the indexing model from the issue text.