Rename template_content to content

`template_content` is a template that hasn't had its placeholders filled
in whereas content is where it would have (which is what has happened
above)

tests/app/broadcast_message/test_rest.py

@@ -35,7 +35,7 @@ def test_get_broadcast_message(admin_request, sample_broadcast_service):
 
     assert response['id'] == str(bm.id)
     assert response['template_id'] == str(t.id)
-    assert response['template_content'] == 'This is a test'
+    assert response['content'] == 'This is a test'
     assert response['template_name'] == t.name
     assert response['status'] == BroadcastStatusType.DRAFT
     assert response['created_at'] is not None