**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
dag_runs = wwwutils.sorted_dag_runs(query, ordering=dag.timetable.run_ordering, limit=num_runs)
            encoded_runs = [wwwutils.encode_dag_run(dr) for dr in dag_runs]
            data = {
                "groups": dag_to_grid(dag, dag_runs, session),
                "dag_runs": encoded_runs,
                "ordering": dag.timetable.run_ordering,
            }
        
        return (
            htmlsafe_json_dumps(data, separators=(",", ":"), dumps=flask.json.dumps),
            {"Content-Type": "application/json; charset=utf-8"},
        )
    @expose("/object/next_run_datasets/<string:dag_id>")
    @auth.has_access([(permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG)])
    def next_run_datasets(self, dag_id):
        """Returns datasets necessary, and their status, for the next dag run"""
        dag = get_airflow_app().dag_bag.get_dag(dag_id)
        if not dag:
            return {"error": f"can't find dag {dag_id}"}, 404
        with create_session() as session:
            data = [
                dict(info)
                for info in session.query(
                    DatasetModel.id,
                    DatasetModel.uri,
                    func.max(DatasetEvent.timestamp).label("lastUpdate"),
                )
                .join(DagScheduleDatasetReference, DagScheduleDatasetReference.dataset_id == DatasetModel.id)
                .join(
                    DatasetDagRunQueue,