**Apache Airflow version**: 2.1.0

**Browsers**: Chrome and Firefox

**What happened**:

Before upgrading to 2.1.0

![before](https://user-images.githubusercontent.com/14293802/120359517-c1ca1100-c2d5-11eb-95ba-58ccc0a3ac37.png)

After upgrading to 2.1.0

![after](https://user-images.githubusercontent.com/14293802/120359528-c4c50180-c2d5-11eb-8e04-f34846ea2736.png)

**What you expected to happen**:

Show/Edit/Delete under Security -> Users are available

<!-- What do you think went wrong? -->

**How to reproduce it**:

Go to Security -> Users (as an admin of course) 
airflow/www/views.py
"""Duration calculation."""
        end_date = self.get('end_date')
        duration = self.get('duration')
        if end_date and duration:
            return timedelta(seconds=duration)
        return None
    formatters_columns = {
        'dag_id': wwwutils.dag_link,
        'task_id': wwwutils.task_instance_link,
        'start_date': wwwutils.datetime_f('start_date'),
        'end_date': wwwutils.datetime_f('end_date'),
        'execution_date': wwwutils.datetime_f('execution_date'),
        'reschedule_date': wwwutils.datetime_f('reschedule_date'),
        'duration': duration_f,
    }
class TaskInstanceModelView(AirflowModelView):
    """View to show records from TaskInstance table"""
    route_base = '/taskinstance'
    datamodel = AirflowModelView.CustomSQLAInterface(models.TaskInstance)  
    class_permission_name = permissions.RESOURCE_TASK_INSTANCE
    method_permission_name = {
        'list': 'read',
        'action_clear': 'edit',
        'action_set_running': 'edit',
        'action_set_failed': 'edit',