Update CKEditor

 - update CKEditor to the latest version (4.4.6) with fixes and security
   updates to prevent attacks such as XSS
 - remove unused plugins from ckeditor such as spellchecking based on
   thrid-party websites and about plugin
 - enable browser spellchecking (for supported browsers)
 - new "bootstrap" theme: flatter and closer to the look and feel of indico
 - move the custom configuration from the ckeditor folder (which is overwritten
   on updates) to RichText.js, including the fix for #1622