Update get_template_by_id_response & post_template_preview_response schemas

To check the format of postage. Neither of these two schemas are used
for validating - they seem to be added for reference.

app/v2/template/template_schemas.py

@@ -37,7 +37,7 @@ get_template_by_id_response = {
         "body": {"type": "string"},
         "subject": {"type": ["string", "null"]},
         "name": {"type": "string"},
-        "postage": {"type": "string"}
+        "postage": {"type": "string", "format": "postage"}
     },
     "required": ["id", "type", "created_at", "updated_at", "version", "created_by", "body", "name"],
 }
@@ -65,7 +65,7 @@ post_template_preview_response = {
         "version": {"type": "integer"},
         "body": {"type": "string"},
         "subject": {"type": ["string", "null"]},
-        "postage": {"type": "string"},
+        "postage": {"type": "string", "format": "postage"},
         "html": {"type": ["string", "null"]},
     },
     "required": ["id", "type", "version", "body"],

tests/app/v2/template/test_template_schemas.py

@@ -35,6 +35,7 @@ valid_json_get_response_with_optionals = {
     'body': 'some body',
     'subject': "some subject",
     'name': 'some name',
+    'postage': 'first',
 }
 
 valid_request_args = [{"id": str(uuid.uuid4()), "version": 1}, {"id": str(uuid.uuid4())}]
@@ -80,6 +81,7 @@ valid_json_post_response_with_optionals = {
     'version': 1,
     'body': "some body",
     'subject': 'some subject',
+    'postage': 'second',
     'html': '<p>some body</p>',
 }