Upgraded to CKEditor 4.0.0

This brings with it some removal of configuration overrides (in
config.js) due to our previous settings now being default with CKEditor.

A new plugin has also been added, MediaEmbed, which lets you easily add
YouTube videos. "Paste From Word" has also been removed, instead just
use "Paste as Plaintext".
Enable CKEditor forcePasteAsPlainText by default