Related task: T222292: Add search box for choosing categories to include in the Campaign settings

A 'depth' option should be added for each individual category that is chosen on the Create/Update Campaign pages.

This setting will be used when generating the list of images for a Participate session.

Note: This task is for the next phase (MVP). More details and subtasks will be added to for the logic used to extract category contents with depth (including suggested APIs/tools to use)

Check https://www.mediawiki.org/w/api.php?action=help&modules=categorytree