**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
GRAPH_ENDPOINT = '/graph?dag_id={dag_id}'.format(
        dag_id=ViewWithDateTimeAndNumRunsAndDagRunsFormTester.DAG_ID
    )
    @classmethod
    def setUpClass(cls):
        super().setUpClass()
    def setUp(self):
        super().setUp()
        self.tester = ViewWithDateTimeAndNumRunsAndDagRunsFormTester(
            self, self.GRAPH_ENDPOINT)
        self.tester.setup()
    def tearDown(self):
        self.tester.teardown()
        super().tearDown()
    @classmethod
    def tearDownClass(cls):
        super().tearDownClass()
    def test_dt_nr_dr_form_default_parameters(self):
        self.tester.test_with_default_parameters()
    def test_dt_nr_dr_form_with_execution_date_parameter_only(self):
        self.tester.test_with_execution_date_parameter_only()
    def test_dt_nr_dr_form_with_base_date_and_num_runs_parameters_only(self):
        self.tester.test_with_base_date_and_num_runs_parameters_only()
    def test_dt_nr_dr_form_with_base_date_and_num_runs_and_execution_date_outside(self):
        self.tester.test_with_base_date_and_num_runs_and_execution_date_outside()
    def test_dt_nr_dr_form_with_base_date_and_num_runs_and_execution_date_within(self):
        self.tester.test_with_base_date_and_num_runs_and_execution_date_within()
class TestGanttView(TestBase):