**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

airflow/www/views.py
from airflow.utils.dates import infer_time_unit, scale_time_units
from airflow.utils.helpers import alchemy_to_dict
from airflow.utils.log.log_reader import TaskLogReader
from airflow.utils.session import create_session, provide_session
from airflow.utils.state import State
from airflow.www import utils as wwwutils
from airflow.www.decorators import action_logging, gzipped, has_dag_access
from airflow.www.forms import (
    ConnectionForm, DagRunForm, DateTimeForm, DateTimeWithNumRunsForm, DateTimeWithNumRunsWithDagRunsForm,
)
from airflow.www.widgets import AirflowModelListWidget
PAGE_SIZE = conf.getint('webserver', 'page_size')
FILTER_TAGS_COOKIE = 'tags_filter'
FILTER_STATUS_COOKIE = 'dag_status_filter'
def get_safe_url(url):
    """Given a user-supplied URL, ensure it points to our web server"""
    valid_schemes = ['http', 'https', '']
    valid_netlocs = [request.host, '']
    parsed = urlparse(url)
    if parsed.scheme in valid_schemes and parsed.netloc in valid_netlocs:
        return url
    return url_for('Airflow.index')
def get_date_time_num_runs_dag_runs_form_data(www_request, session, dag):
    """Get Execution Data, Base Date & Number of runs from a Request"""
    date_time = www_request.args.get('execution_date')
    if date_time:
        date_time = timezone.parse(date_time)
    else:
        date_time = dag.get_latest_execution_date(session=session) or timezone.utcnow()
    base_date = www_request.args.get('base_date')