[WIP] Refactor get_job_listings() to a class

[jom]

Changes proposed in this Pull Request:

• Moves majority of get_job_listings() into two classes:

  • Base class: WP_Job_Manager_Listing_Query_Generator
  • WPJM class: WP_Job_Manager_Query_Generator

• Functionality should be maintained as much as possible. There are a few minor changes. get_job_listings() tests seem to pass.

• Ideally, we'll use WP_Job_Manager_Listing_Query_Generator in Resume Manager to benefit from all the shared code that was in get_job_listings() and get_resumes().

• I was initially thinking this for 1.34.1, but it seems like too big of a change.

• To do:

  • Tests
  • Explore Use $query_args When Determining Random Ordering #1558 options.
  • Deprecate old global functions that are no longer used. Check for overrides and warn developers if they exist.

[tripflex]

Adding myself to this as well just to stay in the loop please