**Description**
In the trigger page, there is a text area to enter (optional) conf json. It would be great if a sample JSON can be provided programatically while defining DAG

**Use case / motivation**

This will improve usability of the UI that triggers a DAG

tests/www/test_views.py
"""
        Tests view with base_date and num_runs and execution-date URL parameters.
        Scenario: change the base date and num runs and press "Go",
        the selected execution date is outside the new range.
        Should only show dag runs older than base_date in the drop down.
        Should select the latest dag run within the range.
        Should set base date and num runs to submitted values.
        """
        response = self.test.client.get(
            self.endpoint
            + '&base_date={}&num_runs=42&execution_date={}'.format(
                self.runs[1].execution_date.isoformat(), self.runs[0].execution_date.isoformat()
            ),
            data=dict(username='test', password='test'),
            follow_redirects=True,
        )
        self.test.assertEqual(response.status_code, 200)
        data = response.data.decode('utf-8')
        self.assert_base_date_and_num_runs(self.runs[1].execution_date, 42, data)
        self.assert_run_is_not_in_dropdown(self.runs[0], data)
        self.assert_run_is_selected(self.runs[1], data)
        self.assert_run_is_in_dropdown_not_selected(self.runs[2], data)
        self.assert_run_is_in_dropdown_not_selected(self.runs[3], data)
    def test_with_base_date_and_num_runs_and_execution_date_within(self):