Publish: ex/importing subscribers

[Fang-]

Moving publish notebooks across eras works nicely. Posts and comments get moved over. The only thing that doesn't get ported are subscribers.

Of course, we can't move the subscriptions themselves across eras, but we can store the list of subscribed ships, and then send out invites to them post-import.

[ohAitch]

And perhaps conversely the list of outbound subscriptions, so you know which invites to auto-accept? Technically, everything is a noun, and can be moved across eras ;)

[ixv]

Closing this as it will be addressed by @liam-fitzgerald's more general breach handling work.