message: shift to custom slate-md-serializer
diff: "slate": "^0.33.4",
  "slate-react": "^0.12.4",
  "slate-html-serializer": "^0.6.1",
  - "slate-md-serializer": "^3.1.0",
  + "slate-md-serializer": "matrix-org/slate-md-serializer#50a133a8",
  "sanitize-html": "^1.14.1",
  "text-encoding-utf-8": "^1.0.1",
  "url": "^0.11.0",
  