message: Restore `Field` value getter for `RegistrationForm`
  When working on the `Field` validation support, I thought `RegistrationForm`'s
  refs would be okay to leave as is, but I missed that they also depended on the
  value getter.
  For the moment, it's quicker to temporarily revive the value getter to get
  registration working.
  Fixes
diff: @@ -53,6 +53,12 @@ export default class Field extends React.PureComponent {
  };
  }
  + /* TODO: Remove this once `RegistrationForm` no longer uses refs */
  + get value() {
  + if (!this.refs.fieldInput) return null;
  + return this.refs.fieldInput.value;
  + }
  +
  onChange = (ev) => {
  if (this.props.onValidate) {
  const result = this.props.onValidate(ev.target.value);
  