**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
"""Get Execution Data, Base Date & Number of runs from a Request"""
    date_time = www_request.args.get("execution_date")
    run_id = www_request.args.get("run_id")
    
    if run_id:
        dagrun = dag.get_dagrun(run_id=run_id, session=session)
        date_time = dagrun.execution_date
    elif date_time:
        date_time = _safe_parse_datetime(date_time)
    else:
        date_time = dag.get_latest_execution_date(session=session) or timezone.utcnow()
    base_date = www_request.args.get("base_date")
    if base_date:
        base_date = _safe_parse_datetime(base_date)
    elif run_id:
        base_date = (timezone.utcnow() + timedelta(seconds=1)).replace(microsecond=0)
    else:
        
        
        base_date = (date_time + timedelta(seconds=1)).replace(microsecond=0)
    default_dag_run = conf.getint("webserver", "default_dag_run_display_number")
    num_runs = www_request.args.get("num_runs", default=default_dag_run, type=int)