BUG Builds on 6e34f65314 (ref issue #125)

- 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 is redirected to a GridField with a “Forbidden” error

Added logic to determine if last action a user ran as part of a
WorkflowInstance, who would normally be without canView() on a
WorkflowInstance at this point, can view it.

- Added getMostRecentActionForUser() method.
- Added tests to support WorkflowInstance#getMostRecentActionForUser().
- Added tests to support WorkflowInstance#canView().