Fix the slightly incomplete logic for changing private story permissions
Fix the slightly incomplete logic for changing private story permissions

This commit fixes the validation to be less overzealous. Previously
it was impossible to remove the last team from a story in the same
PUT request as adding users when the permissions previously had no
users, and vice versa.

Change-Id: I16a7451e8c24da352357abb0590d0ace5b4e77f4

ColdrickSotK/storyboard,ColdrickSotK/storyboard,ColdrickSotK/storyboard