Choose default state of Job Type checkboxes - selected or deselected.

Some users may have a bunch of different Job Types on their site, which can be tedious for users to have to deselect most of them to make their job search more granular. It would be nice to add an option in Settings for site administrators, so the Job Types can be globally selected or deselected. It could go right here:

Alternatively, an option for the _candidate_ to check or uncheck all on the search form itself would be useful.

Ref. 942036-zen and [forums post](https://wordpress.org/support/topic/job-categorie-checkboxes/).

Start by locating the Job Types search form and the WordPress admin Settings screen referenced in the issue. Determine whether the implementation should support an administrator-controlled default, a candidate-facing select-all option, or both; done means the chosen behavior works consistently for job searches and is configurable where intended.

Written by the indexing model from the issue text.