**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
x_axis_date=True,
            x_axis_format=LINECHART_X_AXIS_TICKFORMAT,
            height=chart_height,
            chart_attr=self.line_chart_attr,
        )
        tis = dag.get_task_instances_before(base_date, num_runs, session=session)
        for task in dag.tasks:
            y_points = []
            x_points = []
            for ti in tis:
                if ti.task_id != task.task_id:
                    continue
                dttm = wwwutils.epoch(ti.execution_date)
                x_points.append(dttm)
                
                y_points.append(ti.prev_attempted_tries)
            if x_points:
                chart.add_serie(name=task.task_id, x=x_points, y=y_points)
        tries = sorted({ti.try_number for ti in tis})
        max_date = max(ti.execution_date for ti in tis) if tries else None
        chart.create_y_axis("yAxis", format=".02f", custom_format=False, label="Tries")
        chart.axislist["yAxis"]["axisLabelDistance"] = "-15"
        session.commit()
        form = DateTimeWithNumRunsForm(
            data={
                "base_date": max_date or timezone.utcnow(),
                "num_runs": num_runs,
            }
        )
        chart.buildcontent()