in studio fix latex source edit to save source_code metadata properly

metadata is being managed by the new backbone code, and thus saving of `source_code` in the Studio latex2edx source editor must be done through that framework.

Note that `source_code` is multiline text, whereas `CMS.Views.Metadata.String` uses a `<input type="text">` which cannot accept multiline text.  Thus the replacement with textarea, done in source-edit.