Support Reorganizing and Optionally Hiding Items in Navigation Bar

[breezewish]

As title described. This feature enables domain owners to have more control with the domain. For example, domain owners like university TAs may not need the contest feature.

[twd2]

To hide items, just remove the permissions.

[breezewish]

Usually user wants a unified place to do these jobs. It is not friendly to adjust order at one place and hide some item in another place since both of them are editing items. In addition, the permission is not exactly the same as the visibility. By talking about the permission, it is about whether specific user could do specific things. This proposal is about whether a specific group of functionality is disabled for a domain.