**Description**

Airflow DAG Run UI (Browse -> DAG Runs) is really useful for managing many DAG Runs, creating specific DAG Runs and so on.
It would be great to have an additional option to "With Selected DAG Runs" -> "Clear" to reset all task instances under those DAG Runs.

**Use case / motivation**

When rerunning DAGs especially during development cycles it is tedious to go into the Tree view and clear many DAG Runs manually. It would be great to have some way to do this in bulk / over a range of DAG Runs. 

The DAG Run UI seems like a good place to add this since it is already a kind of administrative area over all the DAG Runs and has "with selected" options like "delete", "mark failed", etc. Discussion welcome :)

**Related Issues**
None

tests/www/test_views.py
dag_id = 'example_bash_operator'
        test_dag_id = "non_existent_dag"
        DM = models.DagModel
        dag_query = self.session.query(DM).filter(DM.dag_id == dag_id)
        dag_query.first().tags = []  
        self.session.commit()
        dag_query.update({'dag_id': test_dag_id})
        self.session.commit()
        resp = self.client.get('/', follow_redirects=True)
        self.check_content_in_response('/delete?dag_id={}'.format(test_dag_id), resp)
        self.check_content_in_response("return confirmDeleteDag(this, '{}')".format(test_dag_id), resp)
        self.session.query(DM).filter(DM.dag_id == test_dag_id).update({'dag_id': dag_id})
        self.session.commit()
    @parameterized.expand(["graph", "tree"])
    def test_show_external_log_redirect_link_with_local_log_handler(self, endpoint):
        """Do not show external links if log handler is local."""
        url = f'{endpoint}?dag_id=example_bash_operator'
        with self.capture_templates() as templates:
            self.client.get(url, follow_redirects=True)
            ctx = templates[0].local_context
            self.assertFalse(ctx['show_external_log_redirect'])
            self.assertIsNone(ctx['external_log_name'])
    @parameterized.expand(["graph", "tree"])
    @mock.patch('airflow.utils.log.log_reader.TaskLogReader.log_handler', new_callable=PropertyMock)
    def test_show_external_log_redirect_link_with_external_log_handler(self, endpoint, mock_log_handler):
        """Show external links if log handler is external."""
        class ExternalHandler(ExternalLoggingMixin):