code_mirror: move editor field's inline css to separate file

This was not valid HTML5:

  AssertionError: Using nu validator to parse the view "SQLMethod_view" (from erp5_core bt) with warnings displayed :
  Error: line 782 column 23 : Element “style” not allowed as child of element “div” in this context. (Suppressing further errors from this subtree.): </script>
  <style type="text/css">
    .cm
  Error: line 822 column 23 : Element “style” not allowed as child of element “div” in this context. (Suppressing further errors from this subtree.): script>

  <style type="text/css">
    .max