**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
"""Redirect from url param."""
        return redirect(url_for("Airflow.duration", **sanitize_args(request.args)))
    @expose("/dags/<string:dag_id>/duration")
    @auth.has_access(
        [
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),
        ]
    )
    @action_logging
    @provide_session
    def duration(self, dag_id, session=None):
        """Get Dag as duration graph."""
        dag = get_airflow_app().dag_bag.get_dag(dag_id, session=session)
        dag_model = DagModel.get_dagmodel(dag_id, session=session)
        if not dag:
            flash(f'DAG "{dag_id}" seems to be missing.', "error")
            return redirect(url_for("Airflow.index"))
        wwwutils.check_import_errors(dag.fileloc, session)
        wwwutils.check_dag_warnings(dag.dag_id, session)
        default_dag_run = conf.getint("webserver", "default_dag_run_display_number")
        base_date = request.args.get("base_date")
        num_runs = request.args.get("num_runs", default=default_dag_run, type=int)
        if base_date:
            base_date = _safe_parse_datetime(base_date)