To make it easier to subclass WP_REST_Controller's children and reuse their methods, I propose we introduce four protected class variables:

• $namespace
• $rest_base
• $plural_label
• $singular_label

Edit: As it turns out, we can't use variables in the labels :(

Related #2056

We'll need to update:

• WP_REST_Attachments_Controller
• WP_REST_Comments_Controller
• WP_REST_Meta_Controller
• WP_REST_Post_Statuses_Controller
• WP_REST_Post_Types_Controller
• WP_REST_Posts_Controller
• WP_REST_Revisions_Controller
• WP_REST_Taxonomies_Controller
• WP_REST_Terms_Controller
• WP_REST_Users_Controller