**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 conf.getboolean("webserver", "SHOW_RECENT_STATS_FOR_COMPLETED_RUNS", fallback=True):
            last_dag_run = (
                session.query(DagRun.dag_id, sqla.func.max(DagRun.execution_date).label("execution_date"))
                .join(DagModel, DagModel.dag_id == DagRun.dag_id)
                .filter(DagRun.state != State.RUNNING, DagModel.is_active)
                .group_by(DagRun.dag_id)
            )
            last_dag_run = last_dag_run.filter(DagRun.dag_id.in_(filter_dag_ids))
            last_dag_run = last_dag_run.subquery("last_dag_run")
            
            
            last_task_instance_query_result = (
                session.query(
                    TaskInstance.dag_id.label("dag_id"),
                    TaskInstance.state.label("state"),
                    sqla.literal(False).label("is_dag_running"),
                )
                .join(TaskInstance.dag_run)
                .join(
                    last_dag_run,
                    and_(
                        last_dag_run.c.dag_id == TaskInstance.dag_id,
                        last_dag_run.c.execution_date == DagRun.execution_date,
                    ),
                )
            )
            final_task_instance_query_result = union_all(
                last_task_instance_query_result, running_task_instance_query_result
            ).alias("final_ti")
        else: