Add vidyo option to append text to booking emails

 - Add an option to the vidyo plugin to allow admins to add HTML content at
   the bottom of vidyo room booking confirmation emails
 - fix broken ckEditor in plugin options
 - fix #1650
I can change the selector but I'm not sure I can give it an id, since this is content in a tab and there are multiple tabs (each one having that button)
Or generate an id based on the tab name if you think it is better, but I am not sure it is unique....