**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"
    resp = admin_client.get(f"trigger?dag_id={test_dag_id}&origin={test_origin}")
    check_content_in_response(f'<a class="btn" href="{expected_origin}">Cancel</a>', resp)
@pytest.mark.parametrize(
    "request_conf, expected_conf",
    [
        (None, {"example_key": "example_value"}),
        ({"other": "test_data", "key": 12}, {"other": "test_data", "key": 12}),
    ],
)
def test_trigger_dag_params_conf(admin_client, request_conf, expected_conf):