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
That is cleaner, however if the name is full of dots (it's the full python class path) so I can't do the above, I have to do something like:
```
$('#${ name.replace(".", "\\\\.") }').val(editor.get());
```
And I have to do it in a couple more places, so I don't know if it is actually nicer...