Public API with `per_page=0` can cause memory overflow

[ajolipa]

Currently we allow this parameter to be set to zero, which then returns all records in one response, without checking whether the amount of data being transmitted is reasonable. Past a certain quantity of data in the model this can end up causing the Heroku memory quota to be exceeded and the request to fail. It seems we should guard against this somehow, although since it seems like we'd need to do the database query to figure out whether or not it was too much data I'm not certain exactly how to do it.