Better variable elements

[Sitebase]

If we use a span around each variable where we attach data-content that contains the text you want to use in the WYSIWYG. We can use a content replace using just CSS. This way we don't have to remove/add nodes while switching between visual and source mode. Because once send, the email client doesn't have this special CSS replace resulting in the actual variable value being shown.

POC:
https://jsfiddle.net/sitebase/ymhv33jz/5/

This will also solve some other issues like:
#2