**Description**

For a DAG that looks like this:

```
A >> B
```

If A fails, B goes into "upstream_failed" state. If a user then marks A "success", B will remain in "upstream_failed" state. It will not automatically start running. This scenario often happens if the failure of A is dealt with outside of Airflow and the user does not want Airflow to run A again. But he usually expect Airflow to run B after A is marked "success".

**Use case / motivation**

After A is marked "success", its downstream tasks B should be cleared automatically and get ready to be scheduled. To avoid changing this behaviour completely, there can be a toggle button next to "Success" that lets the user decide if he wants Airflow to automatically clear downstream tasks that are in "upstream_failed" state.
airflow/www/views.py
"""List loaded plugins."""
        plugins_manager.ensure_plugins_loaded()
        plugins_manager.integrate_executor_plugins()
        plugins_manager.initialize_extra_operators_links_plugins()
        plugins_manager.initialize_web_ui_plugins()
        plugins = []
        for plugin_no, plugin in enumerate(plugins_manager.plugins, 1):
            plugin_data = {
                'plugin_no': plugin_no,
                'plugin_name': plugin.name,
                'attrs': {},
            }
            for attr_name in self.plugins_attributes_to_dump:
                attr_value = getattr(plugin, attr_name)
                plugin_data['attrs'][attr_name] = attr_value
            plugins.append(plugin_data)
        title = "Airflow Plugins"
        doc_url = get_docs_url("plugins.html")
        return self.render_template(
            'airflow/plugin.html',
            plugins=plugins,
            title=title,
            doc_url=doc_url,
        )
class PoolModelView(AirflowModelView):