Change Scraper.checker_ref_url max_length to 500

The field `Scraper.checker_ref_url` was defined as `URLField(blank=True)` it lacked a `max_length` argument which according to Django docs would default the maximum length of the field to 200 characters, however my application requires at least 370. So this is an update to put it at 500 characters to be on the safe side.