MDL-75078 editor_tiny: Disable Tiny template plugin

The template plugin is a part of Tiny, and requires us to create an
administrator UI to configure the templates.

This requires much more thought on workflows, intent, and so on so we'll
park it for later.

Part of MDL-75966.

lib/editor/tiny/classes/manager.php

@@ -490,6 +490,9 @@ class manager {
             // These are not generally compatible with Moodle.
             'image',
             'media',
+
+            // Disable the Template plugin for now.
+            'template',
         ];
     }