message: Re-enable register button on change to working HS
  Register button disabling is done via serverErrorIsFatal so we need
  to reset this on a successful validation.
diff: @@ -180,7 +180,10 @@ module.exports = React.createClass({
  serverConfig.hsUrl,
  serverConfig.isUrl,
  );
  - this.setState({serverIsAlive: true});
  + this.setState({
  + serverIsAlive: true,
  + serverErrorIsFatal: false,
  + });
  } catch (e) {
  this.setState({
  busy: false,
  