Actualize schema to WP custom post types and taxonomies

* Drop previous tables
* Add questions
Redirect flourish models to WP tables

* Define WP-related constants
Add WordPressFacade to split up WP related into one place
Add WordPress entities registrator on init hook

* Implement post type and taxonomies in registrator
* Generate labels in entity registrator
Merge branch 'issue/add-simple-references-through-custom-post-type' into develop