remove broadcast_data from SerialisedTemplate.ALLOWED_PROPERTIES

ALLOWED_PROPERTIES is a list containing fields that it will attempt to
deserialize into the returned object. If a field isn't present on the
underlying data source (whether that's a dump from a marshmallow schema,
a blob retrieved from redis, or anything else), then SerialisedModel
will raise an exception. However, SerialisedModel isn't involved when
setting the cache, so with that said the correct flow for adding a
column to a cached database model:

PR #1

* add new column to DB
* add new column to model, and to template_schema (because that schema
  is used to create the dict that goes in to redis). New redis keys
  start getting populated.

Deploy that through, and then clear redis

PR #2

* add new column to SerialisedTemplate.ALLOWED_PROPERTIES. this means
  that it'll start reading that value from redis
* now instances of the app will always have the new field in their
  template objects, whether they came from database directly or from
  the cache

This commit removes the field from ALLOWED_PROPERTIES. After it's
deployed we'll be able to clear redis, observe redis being populated
with the new field, and then we'll be able to re-add it to
ALLOWED_PROPERTIES, ready to use.

app/serialised_models.py

@@ -47,7 +47,6 @@ class SerialisedTemplate(SerialisedModel):
         'subject',
         'template_type',
         'version',
-        'broadcast_data',
     }
 
     @classmethod