message: Keep sign in button disabled on success
  This changes password login to keep the sign in button locked when proceeding
  successfully to avoid the possibility of double device sign in.
  Part of
diff: @@ -245,6 +245,7 @@ export default createReactClass({
  }
  this.setState({
  + busy: false,
  errorText: errorText,
  // 401 would be the sensible status code for 'incorrect password'
  // but the login API gives a 403 https://matrix.org/jira/browse/SYN-744
  @@ -252,13 +253,6 @@ export default createReactClass({
  // We treat both as an incorrect password
  loginIncorrect: error.httpStatus === 401 || error.httpStatus === 403,
  });
  - }).finally(() => {
  - if (this._unmounted) {
  - return;
  - }
  - this.setState({
  - busy: false,
  - });
  });
  },
  