add broadcast to template_types and add broadcast_data

had to go through the code and change a few places where we filter on
template types. i specifically didn't worry about jobs or notifications.

Also, add braodcast_data - a json column that might contain arbitrary
broadcast data that we'll figure out as we go. We don't know what it'll
look like, but it should be returned by the API

tests/app/template/test_rest.py

@@ -14,6 +14,7 @@ from notifications_utils import SMS_CHAR_COUNT_LIMIT
 
 
 from app.models import (
+    BROADCAST_TYPE,
     EMAIL_TYPE,
     LETTER_TYPE,
     SMS_TYPE,
@@ -31,6 +32,7 @@ from tests.conftest import set_config_values
 
 
 @pytest.mark.parametrize('template_type, subject', [
+    (BROADCAST_TYPE, None),
     (SMS_TYPE, None),
     (EMAIL_TYPE, 'subject'),
     (LETTER_TYPE, 'subject'),
@@ -529,6 +531,7 @@ def test_should_get_return_all_fields_by_default(
     )
     assert json_response['data'][0].keys() == {
         'archived',
+        'broadcast_data',
         'content',
         'created_at',
         'created_by',