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Decouple UndoableCommand from ReadOnlyAddressBook (#861)

The undo/redo mechanism uses an UndoRedoStack stored in the
LogicManager, which uses 2 stacks to store executed and undone
UndoableCommands. The UndoableCommands stores the previousAddressBook
state before execution, which is used to restore the AddressBook state
during an undo command.

This results in unnecessary coupling between the UndoableCommand (within
the Logic Layer) and AddressBook (within the Model Layer).

Let's add a new class, VersionedAddressBook which extends AddressBook
and shift the undo redo machanism from UndoRedoStack in LogicManager to
VersionedAddressBook in ModelManager. This VersionedAddressBook class
will use a list to keep track of the different versions of AddressBook
by maintaining a list of AddressBook States.

Since VersionedAddressBook will store all the AddressBook states,
UndoableCommand will no longer need to store the
previousAddressBookState in itself.

  [1/5] System Tests: remove model verification checks
  [2/5] Add VersionedAddressBook Class
  [3/5] Remove UndoRedoStack class
  [4/5] Remove UndoableCommand class
  [5/5] DeveloperGuide: update section on UndoRedoStack to VersionedAddressBook

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