**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
test_dag_id = "example_bash_operator"
        resp = self.client.get('trigger?dag_id={}&origin={}'.format(test_dag_id, test_origin))
        self.check_content_in_response(
            '<button type="button" class="btn" onclick="location.href = \'{}\'; return false">'.format(
                expected_origin),
            resp)
    def test_trigger_endpoint_uses_existing_dagbag(self):
        """
        Test that Trigger Endpoint uses the DagBag already created in views.py
        instead of creating a new one.
        """
        url = 'trigger?dag_id=example_bash_operator'
        resp = self.client.post(url, data={}, follow_redirects=True)
        self.check_content_in_response('example_bash_operator', resp)
class TestExtraLinks(TestBase):
    def setUp(self):
        from tests.test_utils.mock_operators import Dummy2TestOperator, Dummy3TestOperator
        self.endpoint = "extra_links"
        self.default_date = datetime(2017, 1, 1)
        class RaiseErrorLink(BaseOperatorLink):
            name = 'raise_error'
            def get_link(self, operator, dttm):
                raise ValueError('This is an error')
        class NoResponseLink(BaseOperatorLink):