**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): Kubernetes 1.20.2

**Environment**: Debian Buster image running in Kubernetes 1.20.2

- **Cloud provider or hardware configuration**: Ubuntu on VMware on premise
- **OS** (e.g. from /etc/os-release): Buster
- **Kernel** (e.g. `uname -a`):4.15.0-129-generic
- **Install tools**: Docker with a helm chart

**What happened**:  After enabling an SMTP back end and setting my DAG to notify on failure with

```
'email_on_failure': True,
'email_on_retry': True,
'email': '<my email>'
```

The email is sent and the Mark success portion fails with a 404

**What you expected to happen**: When clicking the Mark Success link, expect the link to mark the DAG as a success and forward on to some where in the Airflow UI


**How to reproduce it**: Enable email on your server and have a DAG that fails. Click the link in the email

As minimally and precisely as possible. Keep in mind we do not have access to your cluster or dags.

Enable an SMTP connection for alerts. Use this DAG to always fail

Failure Dag:

```
from airflow import DAG
from airflow.operators.dummy_operator import DummyOperator
from airflow.operators.bash_operator import BashOperator
from airflow.operators.python_operator import PythonOperator
from airflow.version import version
from datetime import datetime, timedelta

def error_function():
    raise Exception('Something wrong')

default_args = {
    'owner': 'airflow',
    'depends_on_past': False,
    'email_on_failure': True,
    'email_on_retry': True,
    'email': '<your email>',
    'retries': 0,
    'retry_delay': timedelta(minutes=5)
}

with DAG('failure_dag',
         start_date=datetime(2019, 1, 1),
         max_active_runs=3,
         schedule_interval=timedelta(minutes=30),  # https://airflow.apache.org/docs/stable/scheduler.html#dag-runs
         default_args=default_args,
         catchup=False) as dag:

    t0 = PythonOperator(
    task_id='failing_task',
    python_callable=error_function
    )
```
Receive the email and then click the Mark Success link generated from the method [here](https://github.com/apache/airflow/blob/bc5ced3e54b3cf855808e04f09543159fd3fa79f/airflow/models/taskinstance.py#L531-L543)


**Anything else we need to know**:

How often does this problem occur: Every time the link is created in email
Possible Causes: The mark success view requires a POST request but the link produces a GET request.

@ephraimbuddy 
airflow/models/taskinstance.py
from airflow.configuration import conf
from airflow.exceptions import (
    AirflowException,
    AirflowFailException,
    AirflowRescheduleException,
    AirflowSensorTimeout,
    AirflowSkipException,
    AirflowSmartSensorException,
    AirflowTaskTimeout,
)
from airflow.models.base import COLLATION_ARGS, ID_LEN, Base
from airflow.models.log import Log
from airflow.models.taskfail import TaskFail
from airflow.models.taskreschedule import TaskReschedule
from airflow.models.variable import Variable
from airflow.models.xcom import XCOM_RETURN_KEY, XCom
from airflow.plugins_manager import integrate_macros_plugins
from airflow.sentry import Sentry
from airflow.stats import Stats
from airflow.ti_deps.dep_context import DepContext
from airflow.ti_deps.dependencies_deps import REQUEUEABLE_DEPS, RUNNING_DEPS
from airflow.utils import timezone
from airflow.utils.email import send_email
from airflow.utils.helpers import is_container
from airflow.utils.log.logging_mixin import LoggingMixin
from airflow.utils.net import get_hostname
from airflow.utils.operator_helpers import context_to_airflow_vars
from airflow.utils.platform import getuser
from airflow.utils.session import provide_session
from airflow.utils.sqlalchemy import UtcDateTime