**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
import pytest
from flask import Markup, session as flask_session, template_rendered, url_for
from parameterized import parameterized
from werkzeug.test import Client
from werkzeug.wrappers import BaseResponse
from airflow import models, settings, version
from airflow.config_templates.airflow_local_settings import DEFAULT_LOGGING_CONFIG
from airflow.configuration import conf
from airflow.executors.celery_executor import CeleryExecutor
from airflow.jobs.base_job import BaseJob
from airflow.models import DAG, Connection, DagRun, TaskInstance
from airflow.models.baseoperator import BaseOperator, BaseOperatorLink
from airflow.models.renderedtifields import RenderedTaskInstanceFields as RTIF
from airflow.models.serialized_dag import SerializedDagModel
from airflow.operators.bash import BashOperator
from airflow.operators.dummy_operator import DummyOperator
from airflow.settings import Session
from airflow.ti_deps.dependencies_states import QUEUEABLE_STATES, RUNNABLE_STATES
from airflow.utils import dates, timezone
from airflow.utils.log.logging_mixin import ExternalLoggingMixin
from airflow.utils.session import create_session
from airflow.utils.sqlalchemy import using_mysql
from airflow.utils.state import State
from airflow.utils.timezone import datetime
from airflow.utils.types import DagRunType
from airflow.www import app as application
from tests.test_utils.asserts import assert_queries_count
from tests.test_utils.config import conf_vars
from tests.test_utils.db import clear_db_runs
class TemplateWithContext(NamedTuple):