Add feature to mark a template as redacted

Works similarly to the delete template flow, because it’s a destructive,
one-way action.

Not on the edit template page, because it’s not something you want to be
considering every time you’re editing a template. And we saw that people
couldn’t find the delete button when it was on this page.

Adds a bit more CSS for the `dangerous` banner type, because the content
here is quite complicated. Breaking it into a list helps, but the
spacing didn’t look right, so needed some tweaking.

Can ship independently of the code that shows the redaction, but needs
the API first.

app/notify_client/service_api_client.py

@@ -168,6 +168,12 @@ class ServiceAPIClient(NotifyAdminAPIClient):
         endpoint = "/service/{0}/template/{1}".format(service_id, id_)
         return self.post(endpoint, data)
 
+    def redact_service_template(self, service_id, id_):
+        return self.post(
+            "/service/{}/template/{}".format(service_id, id_),
+            {'redact_personalisation': True}
+        )
+
     def get_service_template(self, service_id, template_id, version=None, *params):
         """
         Retrieve a service template.