Fixes #2584 by restoring Refinery::Page#destroy! functionality.
Extracted WYMeditor to refinerycms-wymeditor.

WIP: For some reason some page admin specs are failing when run after core/spec.
Made the custom assets helper spec not clear all registered javascripts and stylesheets before each spec.
Tidy up asset usage/registration in specs to reveal a hidden pagination failure in xhr_paging_spec.rb
Further decouple WYMeditor from the core and replace with APIs.