Skip update selection in task assignment if change is processing

# Problem

Right now, this code below, found in `components/task-assignment`, updates the user selection when it changes.

```didReceiveAttrs() {
let { taskUserId, users } = getProperties(this, 'taskUserId', 'users');
if (users) {
set(this, 'selectedOption', users.findBy('id', taskUserId));
}
},```

However, this updates while the change is still processing, and rolls back if it fails. It'd be preferred if for the update to be skipped if the change is processing. It also fails to roll back when reassignment fails.

## References
#1435