**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
"""Scheduled slots rendering."""
        pool_id = self.get("pool")
        scheduled_slots = self.get("scheduled_slots")
        if pool_id is not None and scheduled_slots is not None:
            url = url_for("TaskInstanceModelView.list", _flt_3_pool=pool_id, _flt_3_state="scheduled")
            return Markup("<a href='{url}'>{scheduled_slots}</a>").format(
                url=url, scheduled_slots=scheduled_slots
            )
        else:
            return Markup('<span class="label label-danger">Invalid</span>')
    formatters_columns = {
        "pool": pool_link,
        "running_slots": frunning_slots,
        "queued_slots": fqueued_slots,
        "scheduled_slots": fscheduled_slots,
    }
    validators_columns = {"pool": [validators.DataRequired()], "slots": [validators.NumberRange(min=-1)]}
def _can_create_variable() -> bool:
    return get_airflow_app().appbuilder.sm.has_access(
        permissions.ACTION_CAN_CREATE, permissions.RESOURCE_VARIABLE
    )
class VariableModelView(AirflowModelView):