Planet only keeps 2 articles per feed

Created by: JonasHelming

Due to the new_feed_option set to "2", the planet only keeps 2 articles from the same feed while it keeps articles from other feeds for over a year. While this was probably introduced for some spam protection, I do not see a good reason to keep this number so low. I therefore suggest to raise this limit, e.g. to 8 and I will create a PR.