Making locale explicitly non-indexed

Making `User.locale` explicitly non-indexed, to avoid any index to be created for this property.

According to https://developers.google.com/appengine/docs/python/ndb/properties such index might be created as needed: with `StringProperty` described as an "Unicode string; up to 500 characters, indexed"

Now, I haven't seen an index using `locale` (in `index.yaml`) during testing, but I think this property can safely be said to be unindexed.