Make Jetpack Publicize opt-in rather than opt-out

WPJM's Job Listing CPT opts into Jetpack's Publicize by default:

https://github.com/Automattic/WP-Job-Manager/blob/master/includes/class-wp-job-manager-post-types.php#L284

The problem is that this automatically posts new Job Listings to Publicize for sites that likely intended to use Publicize for blog posts. When posting a Job Listing from WP Admin, the user may choose not to publicize it, but all posts created from the frontend are publicized by default.

The site owner can opt out of this by using a filter, but I think that this feature should probably be opt-in, given the automatic and all-encompassing nature of Publicize.

cc @jom

Start in includes/class-wp-job-manager-post-types.php around line 284, where the Job Listing CPT opts into Jetpack Publicize. Trace the existing post-type setting and its filter behavior. Done means new Job Listings, including those created from the frontend, are not publicized by default while site owners can explicitly enable the behavior.

Written by the indexing model from the issue text.