User Task is assigned to "peter"
1. Select assignee filed
2. delete assignee name and confirm
--> task will be unclaimed. "The task has been unclaimed." message appears
3. try to claim the task
--> BOOM. "TaskAlreadyClaimedException" appears
- Add remove/unclaim glyphicon also for assignees unlike current user.
- Unclaiming by deleting current assignee name should work as well.