{EMAIL} shortcode: Client-side required based on disable_emailcheck

The `disable_emailcheck` pref now decides whether the email field in the
{EMAIL} shortcode will be required.

See: https://github.com/e107inc/e107/issues/4115#issuecomment-609147815

e107_core/shortcodes/batch/usersettings_shortcodes.php

@@ -194,9 +194,10 @@ class usersettings_shortcodes extends e_shortcode
 		$options = array(
 			'size'     => 40,
 			'title'    => '',
-			'required' => true,
 		);
 
+		if (e107::getPref('disable_emailcheck') == 0) $options['required'] = true;
+
 		if(!empty($sc->var['user_email']) && !empty($sc->var['user_xup'])) // social login active.
 		{
 			$options['readonly'] = true;