Deprecate unpublished Mixtape REST API and enable standard REST API

[jom]

Replaces #1303

While never published officially (has always required define( 'WPJM_REST_API_ENABLED', true ), we will be deprecating this REST API. Our scope and some purpose for the REST API has changed since originally planning it. For now, we'll just be enabling WP core's REST API on our CPT and custom taxonomies.

Primary Impact:

• Settings endpoint ( /wp-json/wpjm/v1/settings) will no longer be available.
• fields attribute will no longer be available in the job-listings endpoints (/wp-json/wp/v2/job-listings/*). We'll eventually use register_meta() to put this in the meta attribute of core's REST API response.

As this was never officially released, I propose doing just a single major release for actually removing Mixtape (1.33.0).

Tasks:

• Throw deprecation notice if site has WPJM_REST_API_ENABLED constant set.
• Optional:
• If that constant is not set, set up the relevant attributes with registering post types and taxonomies.

In 1.33.0...

• Remove Mixtape and its implementation in WPJM.