Change default new_site timestamp 

This is to partially address the @jamiecobbett's concern raised in https://github.com/alphagov/redirector/pull/505 is for sites that may not have valid crawls located in the TNA.

This change of the default tna_timestamp generated by new_site will ensure that if the bouncer configuration is not updated upon the final crawl of the old site, at minimum, the date of any 410 page links will be in the future, and as a result will serve the latest TNA page. 

A minor change with some future-proofing benefit.