Better groups and repeatable WYSIWYG

Handle repeatable WYSIWYG. Now works as repeatable, in a repeatable group, and even repeatable field inside a repeatable group.

This was a little tricky to get working. WordPress was creating the tinyMCE editor, even for the hidden placeholder fields. This makes cloning impossible. To get round this, I inserted the placeholder markup as a JS variable, that we can easily use later to insert the correct markup, and create a new field based on that.

@joehoyle - Take a look and check my implementation is not totally bonkers. Thanks!