**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
"clear",
        lazy_gettext("Clear"),
        lazy_gettext(
            "Are you sure you want to clear the state of the selected task"
            " instance(s) and set their dagruns to the QUEUED state?"
        ),
        single=False,
    )
    @action_has_dag_edit_access
    @provide_session
    @action_logging
    def action_clear(self, task_instances, session=None):
        """Clears the action."""
        try:
            dag_to_tis = collections.defaultdict(list)
            for ti in task_instances:
                dag = get_airflow_app().dag_bag.get_dag(ti.dag_id)
                dag_to_tis[dag].append(ti)
            for dag, task_instances_list in dag_to_tis.items():
                models.clear_task_instances(task_instances_list, session, dag=dag)
            session.commit()
            flash(f"{len(task_instances)} task instances have been cleared")
        except Exception as e:
            flash(f'Failed to clear task instances: "{e}"', "error")
        self.update_redirect()
        return redirect(self.get_redirect())
    @action("muldelete", "Delete", "Are you sure you want to delete selected records?", single=False)
    @action_has_dag_edit_access
    @action_logging
    def action_muldelete(self, items):