**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
func.sum(case((DatasetEvent.id.is_not(None), 1), else_=0)).label("total_updates"),
                )
                .join(DatasetEvent, DatasetEvent.dataset_id == DatasetModel.id, isouter=not has_event_filters)
                .group_by(
                    DatasetModel.id,
                    DatasetModel.uri,
                )
                .order_by(*order_by)
            )
            if has_event_filters:
                count_query = count_query.join(DatasetEvent, DatasetEvent.dataset_id == DatasetModel.id)
            filters = [~DatasetModel.is_orphaned]
            if uri_pattern:
                filters.append(DatasetModel.uri.ilike(f"%{uri_pattern}%"))
            if updated_after:
                filters.append(DatasetEvent.timestamp >= updated_after)
            if updated_before:
                filters.append(DatasetEvent.timestamp <= updated_before)
            query = query.filter(*filters)
            count_query = count_query.filter(*filters)
            query = query.offset(offset).limit(limit)
            datasets = [dict(dataset) for dataset in query.all()]
            data = {"datasets": datasets, "total_entries": count_query.scalar()}
            return (
                htmlsafe_json_dumps(data, separators=(",", ":"), cls=utils_json.WebEncoder),
                {"Content-Type": "application/json; charset=utf-8"},
            )
    @expose("/robots.txt")
    @action_logging
    def robots(self):