**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 json config when the conf URL parameter is passed,
    or if a params dict is passed in the DAG
        1. Conf is not included in URL parameters -> DAG.conf is in textarea
        2. Conf is passed as a URL parameter -> passed conf json is in textarea
    """
    test_dag_id = "example_bash_operator"
    doc_md = "Example Bash Operator"
    if not request_conf:
        resp = admin_client.get(f"trigger?dag_id={test_dag_id}")
    else:
        test_request_conf = json.dumps(request_conf, indent=4)
        resp = admin_client.get(f"trigger?dag_id={test_dag_id}&conf={test_request_conf}&doc_md={doc_md}")
    expected_dag_conf = json.dumps(expected_conf, indent=4).replace('"', "&#34;")
    check_content_in_response(
        f'<textarea class="form-control" name="conf" id="json">{expected_dag_conf}</textarea>',
        resp,
    )
def test_trigger_dag_params_render(admin_client, dag_maker, session, app, monkeypatch):