Bug 1036928 - Allow to set longer time estimages as 60 minutes

Closed the previous pull- working only on this branch now.
We should be able to solve this with some code in this `TaskForm` class. For example, if you add these three lines to the `clean` method at line 77:

```
        admin_time = cleaned_data.get('admin_time')
        if admin_time:
            cleaned_data['execution_time'] = admin_time
```

then that will make the substitution work. The problem then becomes that the form won't validate if a user enters a value that isn't one of the choices, so you'll also need to add some code to override that validation.