Recent changes broke the reverting of unsaved changes to annotations
(AnnotationController.vm.revert()) when the user presses the _Cancel_ button on
the annotation editor. The annotation's privacy was not being restored to its
previously saved value.

We can't set domainModel.permissions when the user changes the privacy of an
annotation that they're editing, because if the user clicks _Cancel_ we need
to revert the privacy back to its previous value (which we've just overwriten).

So instead add a new vm.isPrivate boolean and set that instead:

- vm.isPrivate() method (which returned true or false based on the value of
  domainModel.permissions) is replaced with a vm.isPrivate boolean.

- updateViewModel() now sets vm.isPrivate according to the value of
  domainModel.permissions.

- updatedDomainModel() nows sets domainModel.permissions according to the
  value of vm.isPrivate.

- setPrivacy() nows sets vm.isPrivate instead of domainModel.permissions.

- We no longer change domainModel.permissions when the focused group
  changes (it will continue to hold shared permissions for the previous group,
  but these will be updated by updatedDomainModel() before saving them to the
  server).

  vm.isPrivate doesn't need to be changed when the focused group changes
  because its value is independent of the focused group.

- saveToDrafts() now saves vm.isPrivate instead of vm.isPrivate()

- restoreFromDrafts() now restores vm.isPrivate instead of
  domainModel.permissions