no issue
- fixed API returning "Invalid mobiledoc structure" errors when `mobiledoc:null` is sent in the payload alongside `lexical: '{...}'`
- updated Admin's `posts` and `pages` adapters to always add `?formats=mobiledoc,lexical` because the API doesn't return `lexical` by default
- added `lexical` attribute to Admin's Post model
- updated `lexical-editor` controller and related components to work with `lexical` always being a JSON string rather than a parsed object
- updated `<KoenigLexicalEditor>` to pass through the lexical state string as initial state and wired up the `onChange` prop
