**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
end_date = task_dict["end_date"] or timezone.utcnow()
            task_dict["end_date"] = end_date
            task_dict["start_date"] = task_dict["start_date"] or end_date
            task_dict["state"] = State.FAILED
            task_dict["operator"] = task.operator_name
            task_dict["try_number"] = try_count
            task_dict["extraLinks"] = task.extra_links
            task_dict["execution_date"] = dttm.isoformat()
            task_dict["run_id"] = dag_run_id
            tasks.append(task_dict)
        task_names = [ti.task_id for ti in tis]
        data = {
            "taskNames": task_names,
            "tasks": tasks,
            "height": len(task_names) * 25 + 25,
        }
        session.commit()
        return self.render_template(
            "airflow/gantt.html",
            dag=dag,
            dag_run_id=dag_run_id,
            execution_date=dttm.isoformat(),
            form=form,
            data=data,
            base_date="",
            root=root,
            dag_model=dag_model,
        )
    @expose("/extra_links")
    @auth.has_access(