BUG Builds on 6e34f65314 (ref issue #125)

Modified logic to determine if 1 of last 2 actions were run by
a user on a WorkflowInstance. If user is normally without canView()
on a WorkflowInstance at this point, ensures she can view it.

Added supporting tests also.

- UserX is assigned action from a AssignUsersToWorkflowAction
- UserX runs a transition leading to another AssignUsersToWorkflowAction
- At this point, user+groups are reset on the current WorkflowInstance
- UserX redirected to a GridField with a “Forbidden” error as a result