**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
tags=arg_tags_filter if arg_tags_filter else None,
            ),
            num_runs=num_runs,
            tags=tags,
            owner_links=owner_links_dict,
            state_color=state_color_mapping,
            status_filter=arg_status_filter,
            status_count_all=all_dags_count,
            status_count_active=status_count_active,
            status_count_paused=status_count_paused,
            tags_filter=arg_tags_filter,
            sorting_key=arg_sorting_key,
            sorting_direction=arg_sorting_direction,
            auto_refresh_interval=conf.getint("webserver", "auto_refresh_interval"),
            dataset_triggered_next_run_info=dataset_triggered_next_run_info,
        )
    @expose("/datasets")
    @auth.has_access([(permissions.ACTION_CAN_READ, permissions.RESOURCE_DATASET)])
    def datasets(self):
        """Datasets view."""
        state_color_mapping = State.state_color.copy()
        state_color_mapping["null"] = state_color_mapping.pop(None)
        return self.render_template(
            "airflow/datasets.html",
            state_color_mapping=state_color_mapping,
        )
    @expose("/next_run_datasets_summary", methods=["POST"])
    @auth.has_access([(permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG)])
    @provide_session
    def next_run_datasets_summary(self, session=None):