**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
self.test.assertNotIn(run.execution_date.isoformat(), data)
        self.test.assertNotIn(run.run_id, data)
    def assert_run_is_in_dropdown_not_selected(self, run, data):
        self.test.assertIn('<option value="{}">{}</option>'.format(
            run.execution_date.isoformat(), run.run_id), data)
    def assert_run_is_selected(self, run, data):
        self.test.assertIn('<option selected value="{}">{}</option>'.format(
            run.execution_date.isoformat(), run.run_id), data)
    def test_with_default_parameters(self):
        """
        Tests view with no URL parameter.
        Should show all dag runs in the drop down.
        Should select the latest dag run.
        Should set base date to current date (not asserted)
        """
        response = self.test.client.get(
            self.endpoint, data=dict(
                username='test',
                password='test'), follow_redirects=True)
        self.test.assertEqual(response.status_code, 200)
        data = response.data.decode('utf-8')
        self.test.assertIn('<label class="sr-only" for="base_date">Base date</label>', data)
        self.test.assertIn('<label class="sr-only" for="num_runs">Number of runs</label>', data)
        self.assert_run_is_selected(self.runs[0], data)
        self.assert_run_is_in_dropdown_not_selected(self.runs[1], data)
        self.assert_run_is_in_dropdown_not_selected(self.runs[2], data)
        self.assert_run_is_in_dropdown_not_selected(self.runs[3], data)
    def test_with_execution_date_parameter_only(self):