message: Lock drawer when stack is not on root screen
diff: @@ -129,18 +129,48 @@ const ChatsStack = createStackNavigator({
  defaultNavigationOptions: defaultHeader
  });
  +ChatsStack.navigationOptions = ({ navigation }) => {
  + let drawerLockMode = 'unlocked';
  + if (navigation.state.index > 0) {
  + drawerLockMode = 'locked-closed';
  + }
  + return {
  + drawerLockMode
  + };
  +};
  +
  const ProfileStack = createStackNavigator({
  ProfileView
  }, {
  defaultNavigationOptions: defaultHeader
  });
  +ProfileView.navigationOptions = ({ navigation }) => {
  + let drawerLockMode = 'unlocked';
  + if (navigation.state.index > 0) {
  + drawerLockMode = 'locked-closed';
  + }
  + return {
  + drawerLockMode
  + };
  +};
  +
  const SettingsStack = createStackNavigator({
  SettingsView
  }, {
  defaultNavigationOptions: defaultHeader
  });
  +SettingsStack.navigationOptions = ({ navigation }) => {
  + let drawerLockMode = 'unlocked';
  + if (navigation.state.index > 0) {
  + drawerLockMode = 'locked-closed';
  + }
  + return {
  + drawerLockMode
  + };
  +};
  +
  const ChatsDrawer = createDrawerNavigator({
  ChatsStack,
  ProfileStack,
  