Fix missing or no article publication date

Existing articles with a missing or invalid publication date keep getting their publication date/time reset to the current time. As a result, these articles will be updated every time a crawl occurs and thus keep reappearing at the top of the users subscribed articles.

This fix sets the default publication date for articles to 0 (Unix epoch) if there is a missing or invalid publication date. As articles are now sorted by crawl date/time, there is no need for an article to keep having the publication date/time being set to the current date/time. This also means that while the article continues to have a missing or invalid date/time, it will not continue to be updated as the publication date/time is defaulted to 0.