**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 a summary of datasets, including the datetime they were last updated and how many updates
        they've ever had
        """
        allowed_attrs = ["uri", "last_dataset_update"]
        
        limit = int(request.args.get("limit", 25))
        offset = int(request.args.get("offset", 0))
        order_by = request.args.get("order_by", "uri")
        uri_pattern = request.args.get("uri_pattern", "")
        lstripped_orderby = order_by.lstrip("-")
        updated_after = _safe_parse_datetime(request.args.get("updated_after"), allow_empty=True)
        updated_before = _safe_parse_datetime(request.args.get("updated_before"), allow_empty=True)
        
        limit = 50 if limit > 50 else limit
        uri_pattern = uri_pattern[:4000]
        if lstripped_orderby not in allowed_attrs:
            return {
                "detail": (
                    f"Ordering with '{lstripped_orderby}' is disallowed or the attribute does not "
                    "exist on the model"