Fix and/or test use of `is None` in Bug.isSubscribed and Bug.isSubscribedToDupes

Both Bug.isSubscribed and Bug.isSubscribedToDupes use an `if person is None: return False` pattern to allow use of these methods without a person, presumably to work without a logged in user. This came up in a review of mine for another branch. Either these methods need to remove the check against None and ensure that the calling site never passes in None, or these methods need to be tested for the None case or when dealing with anonymous users. Or perhaps even some combination of the two.