Add job_manager role besides the admin

For WooCommerce there is shop_manager role. For WPJM would be awesome to have also a job_manager role. So that a job_manager can only concern himself with the vacancies, companies, resumes and applications.

Start by comparing WooCommerce’s shop_manager role with WP-Job-Manager’s admin permissions, focusing on vacancies, companies, resumes, and applications. Done means a job_manager role exists and is limited to those WPJM areas; verify each area in the WordPress admin and confirm unrelated admin access is not granted.

Written by the indexing model from the issue text.