Refactor JavaScriptMessagesController controller

- Removes the dependency of the cfg and theme PHP globals.
- Handles the errors when encoding the messages to JSON.

Signed-off-by: Maurício Meneghini Fauth <mauricio@fauth.dev>

test/classes/Controllers/JavaScriptMessagesControllerTest.php

@@ -21,10 +21,6 @@ class JavaScriptMessagesControllerTest extends TestCase
 {
     public function testIndex(): void
     {
-        global $cfg;
-
-        $cfg['GridEditing'] = 'double-click';
-
         ob_start();
         (new JavaScriptMessagesController())();
         $actual = ob_get_contents();