v1.42.1

A security patch found while planning Phase 7: an article taken out of its
last board became public. No migrations.

Security

• Taking an article out of its last board no longer publishes it. An
  article in no board is public to everyone and federates to its author's
  followers, so when the author, an admin or that board's moderator removed
  an article from the only board it was in, a private board's article became
  readable by guests at its permalink and fetchable at /ap/articles/:slug.
  The last board may now be removed only when it is itself federated (public
  and ActivityPub-enabled), which is the case where nothing becomes more
  visible; any other is refused with a message pointing to deletion, and
  neither the article page nor the edit form offers the control. The check
  runs with the article row locked, so two removals cannot race past it.
• Deleting a board checks for articles with the board row locked. The
  count used to run before the delete, so an article linked in between lost
  the board to the cascade and could be left in none.