Recognise that broadcast messages include content

The content attribute needs to be added to the model so we can use it
later.

app/models/broadcast_message.py

@@ -24,6 +24,7 @@ class BroadcastMessage(JSONModel):
         'template_id',
         'template_name',
         'template_version',
+        'content',
         'service_id',
         'created_by',
         'personalisation',

tests/__init__.py

@@ -650,6 +650,7 @@ def broadcast_message_json(
     approved_by_id=None,
     cancelled_by_id=None,
     areas=None,
+    content=None,
 ):
     return {
         'id': id_,
@@ -659,6 +660,7 @@ def broadcast_message_json(
         'template_id': template_id,
         'template_version': 123,
         'template_name': 'Example template',
+        'content': 'This is a test',
 
         'personalisation': {},
         'areas': areas or [

tests/app/models/test_broadcast_message.py

@@ -34,3 +34,14 @@ def test_simple_polygons(fake_uuid):
         # total coordinates
         [51],
     ]
+
+
+def test_content_comes_from_attribute_not_template(fake_uuid):
+    broadcast_message = BroadcastMessage(broadcast_message_json(
+        id_=fake_uuid,
+        service_id=fake_uuid,
+        template_id=fake_uuid,
+        status='draft',
+        created_by_id=fake_uuid,
+    ))
+    assert broadcast_message.content == 'This is a test'