Dry submenu addition

Tested locally and discussed with @Petervw 

Refactored the way the submenu is built in GA. Made the code more readable and removed some duplication.

Also, I've fixed an inconsistency. The menu slug for the extensions page was still `'yst_ga_licences'`. I've changed this to `'yst_ga_extensions'` and made sure the correct slug is collected in `#load_page` and `#create_menu`.

@andizer could you please review?