Add an option to export job listings to .csv

Try to avoid having to use third party plugins to export listings to .csv and create our own feature to export jobs (and all our other custom post types).

Mentioned in 2853187-t

The issue requests CSV export for job listings and other custom post types but names no files, tests, fields, or export scope. Start by locating the plugin's admin listing and custom-post-type entry points, then clarify which records and columns must be exported. Done means the agreed post types can be exported to usable CSV without a third-party plugin.

Written by the indexing model from the issue text.