bugfix: assignment silently failing

[scottmcdonnell]

Problem
When a Supervisor assigns a person not in their department to a task it silently fails. The API returns an empty list [] for the api/actions/persons/<id>/assign call, but the UI updates the assignee for the task and it looks to the user like that task has been successfully assigned. On page reload the assignment "disappears".

Similarly if a Supervisor selects a task in their department, then more tasks including those NOT in their department and assigns them to a user in their team, then the api returns a list of tasks successfully assigned omitting those not assigned for whatever reason. Again the UI shows ALL selected tasks assigned instead of just the successfully assigned and a page reload fixes this.

Solution
Get the assigned task ids back from the api response and only update the tasks for those. Show an error if any failed assignments.