Add visibility controls for lists

[acozine]

As a user, I should be able to set visibility to public or private on each list I create.
Add a radio button (?) to the List view screen for public/private.
Public lists will be searchable.
Private lists will not be searchable.

Will private lists be share-able with a link, or should they be hidden from all users besides the owner?

[val99erie]

Also see story #23