Codemirror Field Improvements

Full once over of textarea_code field

1. Fixes layout of Codemirror field when Gutenberg is active.
2. Support sorting of Codemirror field.
3. Support Codemirror field within groups.
4. Allow using field ids with "/" in the name.

Others

1. Improve styles of the repeatable row remove button.
2. Allow using field ids with "/" in the name for wysiwyg.

Fix Drag Sorting of Groups When Adding Items

1. Fixes issue where new rows which were then sorted did not save on first round.
2. Fixes JS error '$newRowId does not exist`.

Fix `rest_value_cb` field parameter

Fixes undocumented rest_value_cb parameter

Bring in pull request 1212