Modifying the reviewed state of an edit patchset is an illogical action,
and should be disabled.
This change adds an _editLoaded property to gr-diff-view to
conditionally hide the reviewed checkbox. In addition, attempts to
modify the reviewed state are treated as a no-op.
TODO: Further utilize this approach to conditionally show buttons for
modifying an edit patchset. Will be done in a later change.
Bug: Issue 4437
Change-Id: I0ea98e49c5ec66e73f45fef9db5e3849d6e594df
5f1f0c6f4e