**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 that textarea in Trigger DAG UI is pre-populated
    with param value set in DAG.
    """
    account = {"name": "account_name_1", "country": "usa"}
    expected_conf = {"accounts": [account]}
    expected_dag_conf = json.dumps(expected_conf, indent=4).replace('"', "&#34;")
    DAG_ID = "params_dag"
    param = Param(
        [account],
        schema={
            "type": "array",
            "minItems": 1,
            "items": {
                "type": "object",
                "default": account,
                "properties": {"name": {"type": "string"}, "country": {"type": "string"}},
                "required": ["name", "country"],
            },
        },
    )
    with monkeypatch.context() as m:
        with dag_maker(dag_id=DAG_ID, serialized=True, session=session, params={"accounts": param}):
            EmptyOperator(task_id="task1")
        m.setattr(app, "dag_bag", dag_maker.dagbag)
        resp = admin_client.get(f"trigger?dag_id={DAG_ID}")
    check_content_in_response(
        f'<textarea class="form-control" name="conf" id="json">{expected_dag_conf}</textarea>', resp
    )
def test_trigger_endpoint_uses_existing_dagbag(admin_client):