**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
from airflow.models.dagcode import DagCode
        dag = models.DagBag(include_examples=True).get_dag("example_bash_operator")
        DagCode(dag.fileloc, DagCode._get_code_from_file(dag.fileloc)).sync_to_db()
        url = 'code?dag_id=example_bash_operator'
        resp = self.client.get(url)
        self.check_content_not_in_response('Failed to load file', resp)
        self.check_content_in_response('example_bash_operator', resp)
    @conf_vars({("core", "store_dag_code"): "True"})
    def test_code_from_db_all_example_dags(self):
        from airflow.models.dagcode import DagCode
        dagbag = models.DagBag(include_examples=True)
        for dag in dagbag.dags.values():
            DagCode(dag.fileloc, DagCode._get_code_from_file(dag.fileloc)).sync_to_db()
        url = 'code?dag_id=example_bash_operator'
        resp = self.client.get(url)
        self.check_content_not_in_response('Failed to load file', resp)
        self.check_content_in_response('example_bash_operator', resp)
    def test_paused(self):
        url = 'paused?dag_id=example_bash_operator&is_paused=false'
        resp = self.client.post(url, follow_redirects=True)
        self.check_content_in_response('OK', resp)
    def test_failed(self):