Werk #16202: Mega menu: Hide "Customize" menu and menu items for users with no permission

Before this Werk, users without permissions to access the "Customize" menu could still see it in the mega menu, and then when accessing it they would get an error message. This Werk fixes that behaviour by hiding the menu entries to which the user does not have access, or the entire menu in case the user does not have any customisation permissions.

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