v0.12.18: @newsroom.writer: newsletter issues are written here

publish.py now carries newsletter issues as well as articles: admin/content/newsletter/NNN-YYYY-MM-DD.md here becomes admin/content/newsroom/newsletter/ in the borrowed generator, which renders the issue with the desk directives, checks every quote against its article, draws its cover and imports it to articles/newsletter/. An issue number sgit.ai already has stops the run. One more build now follows the last banner pass, because a new issue's cover is first drawn there and the issue page shows its cover and its LinkedIn box only when the cover exists. Tested with a throwaway issue 006 that was not published. A handover pack for the session that will write issue 6 onwards was prepared outside the repository.