**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
"""Show Method Not Allowed on screen for any error in the Webserver"""
    return (
        render_template(
            "airflow/error.html",
            hostname=get_hostname()
            if conf.getboolean("webserver", "EXPOSE_HOSTNAME", fallback=True)
            else "redact",
            status_code=405,
            error_message="Received an invalid request.",
        ),
        405,
    )
def show_traceback(error):
    """Show Traceback for a given error"""
    return (
        render_template(
            "airflow/traceback.html",
            python_version=sys.version.split(" ")[0],
            airflow_version=version,
            hostname=get_hostname()
            if conf.getboolean("webserver", "EXPOSE_HOSTNAME", fallback=True)
            else "redact",
            info=traceback.format_exc()
            if conf.getboolean("webserver", "EXPOSE_STACKTRACE", fallback=True)
            else "Error! Please contact server admin.",
        ),
        500,
    )
class AirflowBaseView(BaseView):