Display broadcasts without a template

At the moment the admin app expects all broadcasts to have a template,
and expects the content of the alert to come from the template.

This commit makes it so those pages can still get a `Template` instance,
but populated with content straight from the `content` field in the
database.

tests/__init__.py

@@ -656,6 +656,7 @@ def broadcast_message_json(
     cancelled_by_id=None,
     areas=None,
     content=None,
+    reference=None,
     template_name='Example template',
 ):
     return {
@@ -667,6 +668,7 @@ def broadcast_message_json(
         'template_version': 123,
         'template_name': template_name,
         'content': content or 'This is a test',
+        'reference': reference,
 
         'personalisation': {},
         'areas': areas or [