message: keep deserialized parts compatible with part api,
  to avoid breakage when passing real parts
diff: @@ -312,7 +312,7 @@ class UserPillPart extends PillPart {
  serialize() {
  const obj = super.serialize();
  - obj.userId = this.resourceId;
  + obj.resourceId = this.resourceId;
  return obj;
  }
  }
  @@ -403,7 +403,7 @@ export class PartCreator {
  case "room-pill":
  return this.roomPill(part.text);
  case "user-pill":
  - return this.userPill(part.text, part.userId);
  + return this.userPill(part.text, part.resourceId);
  }
  }
  