Convert webform_components_form to D8

Committed and pushed to 8.x-4.x.

• As mentioned above I was originally planning to land most of this in #2293945: Convert components to plugins however working on this and plugins at the same time was just too big. I spent quite a bit of time going around in circles!
• Both \Drupal\book\Form\BookAdminEditForm and https://www.drupal.org/node/1876710 provided lots of direction here.
• Obviously still quite a few @todo's here as this is realistically just a first pass.
• theme_webform_components_page() was removed so a follow up was created #2488950: Add back ability for form_builder to easily swap out components form.