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.
Renamed wym_ prefix to visual_editor_ where relevant and moved relevant functionality out to refinerycms-wymeditor.
Added refinerycms-wymeditor to installation templates.
Merge pull request #2567 from refinery/extract_wymeditor

Extracted WYMeditor to refinerycms-wymeditor.