**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
@expose("/trigger", methods=["POST", "GET"])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_CREATE, permissions.RESOURCE_DAG_RUN),
        ]
    )
    @action_logging
    @provide_session
    def trigger(self, session=None):
        """Triggers DAG Run."""
        dag_id = request.values.get("dag_id")
        run_id = request.values.get("run_id")
        origin = get_safe_url(request.values.get("origin"))
        unpause = request.values.get("unpause")
        request_conf = request.values.get("conf")
        request_execution_date = request.values.get("execution_date", default=timezone.utcnow().isoformat())
        is_dag_run_conf_overrides_params = conf.getboolean("core", "dag_run_conf_overrides_params")
        dag = get_airflow_app().dag_bag.get_dag(dag_id)
        dag_orm = session.query(DagModel).filter(DagModel.dag_id == dag_id).first()
        if not dag_orm:
            flash(f"Cannot find dag {dag_id}")
            return redirect(origin)
        if dag_orm.has_import_errors:
            flash(f"Cannot create dagruns because the dag {dag_id} has import errors", "error")
            return redirect(origin)
        recent_runs = (
            session.query(DagRun.conf, func.max(DagRun.execution_date))
            .filter(
                DagRun.dag_id == dag_id,