**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
if not dag_run_id:
            return {"status": "error", "message": "Invalid dag_run_id"}
        dag = get_airflow_app().dag_bag.get_dag(dag_id)
        if not dag:
            return {"status": "error", "message": f"Cannot find DAG: {dag_id}"}
        set_dag_run_state_to_queued(dag=dag, run_id=dag_run_id, commit=confirmed)
        if confirmed:
            return {"status": "success", "message": "Marked the DagRun as queued."}
        else:
            
            all_task_ids = [task.task_id for task in dag.tasks]
            existing_tis = session.query(TaskInstance.task_id).filter(
                TaskInstance.dag_id == dag.dag_id,
                TaskInstance.run_id == dag_run_id,
            )
            completed_tis_ids = [task_id for task_id, in existing_tis]
            tasks_with_no_state = list(set(all_task_ids) - set(completed_tis_ids))
            details = [str(t) for t in tasks_with_no_state]
            return htmlsafe_json_dumps(details, separators=(",", ":"))
    @expose("/dagrun_failed", methods=["POST"])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG_RUN),
        ]
    )
    @action_logging
    def dagrun_failed(self):