message: Remove duplicated conversion from `enableRichText`
  `enableRichText` would convert the editor state between rich and md versions
  explicitly, but then it would also call `createEditorState`, which can do the
  same thing.
  This removes the duplication, and also supplies the right arguments to
  `createEditorState` so it can do its work correctly.
diff: @@ -686,17 +686,14 @@ export default class MessageComposerInput extends React.Component {
  enableRichtext(enabled: boolean) {
  if (enabled === this.state.isRichTextEnabled) return;
  - let editorState = null;
  - if (enabled) {
  - editorState = this.mdToRichEditorState(this.state.editorState);
  - } else {
  - editorState = this.richToMdEditorState(this.state.editorState);
  - }
  -
  Analytics.setRichtextMode(enabled);
  this.setState({
  - editorState: this.createEditorState(enabled, editorState),
  + editorState: this.createEditorState(
  + enabled,
  + this.state.editorState,
  + this.state.isRichTextEnabled,
  + ),
  isRichTextEnabled: enabled,
  }, ()=>{
  this._editor.focus();
  