**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
)
        form = GraphForm(data=dt_nr_dr_data)
        form.execution_date.choices = dt_nr_dr_data["dr_choices"]
        task_instances = {
            ti.task_id: wwwutils.get_instance_with_map(ti, session)
            for ti in dag.get_task_instances(dttm, dttm)
        }
        tasks = {
            t.task_id: {
                "dag_id": t.dag_id,
                "task_type": t.task_type,
                "extra_links": t.extra_links,
                "is_mapped": isinstance(t, MappedOperator),
                "trigger_rule": t.trigger_rule,
            }
            for t in dag.tasks
        }
        if not tasks:
            flash("No tasks found", "error")
        session.commit()
        doc_md = wwwutils.wrapped_markdown(getattr(dag, "doc_md", None))
        task_log_reader = TaskLogReader()
        if task_log_reader.supports_external_link:
            external_log_name = task_log_reader.log_handler.log_name
        else:
            external_log_name = None
        state_priority = ["no_status" if p is None else p for p in wwwutils.priority]
        return self.render_template(
            "airflow/graph.html",
            dag=dag,