Add render for sub-pages as dropdown menus

[LessComplexity]

Review

The control panel shown only main pages and has no functionality to render sub-menus. Sub-menus can be derived from the 'admin_pages' tables from the DB. Specifically Control::getPages() function gets all the pages from the DB, present in the Control controller (can be accessed via API request).

Desired

• Sub-pages will be shown in a sub-menu