**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
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import copy
import itertools
import json
import logging
import math
import socket
import traceback
from collections import defaultdict
from datetime import datetime, timedelta
from json import JSONDecodeError
from typing import Dict, List, Optional, Tuple
from urllib.parse import unquote, urlparse
import lazy_object_proxy
import nvd3
import sqlalchemy as sqla
from flask import (
    Markup, Response, current_app, escape, flash, g, jsonify, make_response, redirect, render_template,