rename template.serialize to serialize_for_v2

make it clear that this is for the public api, and we shouldn't add
fields to it without considering impacts

also add the broadcast_messages relationship on service and template to
the exclude from the marshmallow schemas, so it's not included elsewhere

tests/app/v2/template/test_get_template.py

@@ -44,7 +44,6 @@ def test_get_template_by_id_returns_200(
         'name': expected_name,
         'personalisation': {},
         'postage': postage,
-        'broadcast_data': None,
     }
 
     assert json_response == expected_response

tests/app/v2/template/test_post_template.py

@@ -2,7 +2,7 @@ import pytest
 
 from flask import json
 
-from app.models import EMAIL_TYPE, LETTER_TYPE, SMS_TYPE, TEMPLATE_TYPES
+from app.models import EMAIL_TYPE, LETTER_TYPE, TEMPLATE_TYPES
 from tests import create_authorization_header
 from tests.app.db import create_template