**Description**

Replace the Configuration JSON text-area with a form that has individual input fields for each key & value. This will remove the need for the user to provide a json object. See hashicorp vault's key-value secrets UI for an example of what this might look like:

![Screenshot 2020-09-21 at 12 06 03](https://user-images.githubusercontent.com/17148324/93759844-df04f680-fc02-11ea-8613-f3bcc0b22d09.png)
 
- If present these keys and values should be serialised as json client-side and posted to the trigger-dag API endpoint as the `conf` request-parameter.
- By default no conf request-parameter is sent. Perhaps a checkbox should be used to determine whether conf is sent at all?
- The user may want to add new keys, or remove keys & values that they have previously added.
- [PR-10839](https://github.com/apache/airflow/pull/10839) is pre-populating the form with the DAG's default params. The new form should be pre-populated with the keys & values from `DAG.params`


**Use case / motivation**

Currently when triggering a manual DAG run with user-provided runtime parameters, the user has to provide a correctly formatted JSON object. Filling in a form with keys & values will be a significantly better user experience, and will be less error prone & laborious than handling json in a free-text field.

This builds on the great work already done on enhancing the Trigger-DAG functionality

**Related Issues**

Originating PR: https://github.com/apache/airflow/pull/10839
test_dag_id = "example_bash_operator"
    response = admin_client.post(f"trigger?dag_id={test_dag_id}", data={"conf": "string and not a dict"})
    check_content_in_response("must be a dict", response)
    with create_session() as session:
        run = session.query(DagRun).filter(DagRun.dag_id == test_dag_id).first()
    assert run is None
def test_trigger_dag_wrong_execution_date(admin_client):
    test_dag_id = "example_bash_operator"
    response = admin_client.post(f"trigger?dag_id={test_dag_id}", data={"execution_date": "not_a_date"})
    check_content_in_response("Invalid execution date", response)
    with create_session() as session:
        run = session.query(DagRun).filter(DagRun.dag_id == test_dag_id).first()
    assert run is None
def test_trigger_dag_execution_date_data_interval(admin_client):
    test_dag_id = "example_bash_operator"
    exec_date = timezone.utcnow()
    admin_client.post(f"trigger?dag_id={test_dag_id}", data={"execution_date": exec_date.isoformat()})
    with create_session() as session:
        run = session.query(DagRun).filter(DagRun.dag_id == test_dag_id).first()
    assert run is not None
    assert DagRunType.MANUAL in run.run_id
    assert run.run_type == DagRunType.MANUAL
    assert run.execution_date == exec_date
    
    
    today_midnight = exec_date.replace(hour=0, minute=0, second=0, microsecond=0)
    assert run.data_interval_start == (today_midnight - datetime.timedelta(days=1))
    assert run.data_interval_end == today_midnight
def test_trigger_dag_form(admin_client):