MDL-71585 qbank_managecategories: Add managecategories to core

This implementation will introduce a qbank plugin "managecategories"
which will add the question categories feature in the question bank view
by replacing the core classes. Having this plugin will give users
the flexibility of enabling or disabling the category tab.

lib/classes/plugin_manager.php

@@ -1943,6 +1943,7 @@ class core_plugin_manager {
                 'editquestion',
                 'exportquestions',
                 'importquestions',
+                'managecategories',
                 'viewcreator',
                 'viewquestionname',
                 'viewquestiontext',