MDL-67806 tool_mobile: Add Dark mode option in disabled features

We must allow site admins to remotely disable that feature in the app

admin/tool/mobile/classes/api.php

@@ -433,6 +433,7 @@ class api {
                 '$mmLoginEmailSignup' => new lang_string('startsignup'),
                 'NoDelegate_ForgottenPassword' => new lang_string('forgotten'),
                 'NoDelegate_ResponsiveMainMenuItems' => new lang_string('responsivemainmenuitems', 'tool_mobile'),
+                'NoDelegate_DarkMode' => new lang_string('darkmode', 'tool_mobile'),
             ),
             "$mainmenu" => array(
                 '$mmSideMenuDelegate_mmaFrontpage' => new lang_string('sitehome'),

admin/tool/mobile/lang/en/tool_mobile.php

@@ -51,6 +51,7 @@ App help|https://someurl.xyz/help|inappbrowser
 My grades|https://someurl.xyz/local/mygrades/index.php|embedded|en
 Mis calificaciones|https://someurl.xyz/local/mygrades/index.php|embedded|es
 </pre>';
+$string['darkmode'] = 'Dark mode';
 $string['disabledfeatures'] = 'Disabled features';
 $string['disabledfeatures_desc'] = 'Select here the features you want to disable in the Mobile app for your site. Please note that some features listed here could be already disabled via other site settings. You will have to log out and log in again in the app to see the changes.';
 $string['displayerrorswarning'] = 'Display debug messages (debugdisplay) is enabled. It should be disabled.';