closes https://github.com/TryGhost/Team/issues/3387
- adds syncing of mobiledoc->lexical formats for snippets when opening the beta editor
- this is a one-way sync
- new snippets or changes made to snippets inside the beta will not be available in the old editor
- creating or changing snippets in the old editor will sync to (and potentially overwrite snippets) in the beta editor
- fixed incorrect saving of doubly-escaped JSON when creating snippets in the beta editor
