MOBILE-3022: If both workflow engines are disabled the app should not show the Tasks menu option
MOBILE-2990: Fix uncaught exception 'NSInvalidArgumentException', reason: 'task must not be nil'
MOBILE-2991: Fix crash in - [BaseFileFolderListViewController tableView:cellForRowAtIndexPath:], line 128
MOBILE-2993: Fix NSInvalidArgumentException in - [ProcessTasksCell updateStatusLabelUsingTask:]

Pooled tasks were being rendered in the "Tasks I Started" screen, they don't have an assignee which was causing the crash. Furthermore, a "Reassign" button was also available for this view but self.task is nil as the view is initialised with a process object. This change there also provides a better fix for MOBILE-2990.

Pooled tasks/workflows (Activiti and JBPM) are no longer displayed as there are other rendering issues (missing strings).