prepare to import more notes

`recreateChain` converts a list of notes into a forest of notes, using
notes that are not replies as roots, and replies as child nodes,
recursively.

Previously, notes that are replies to notes not included in the
export, and their children, were never put in the forest, and
therefore wheren't imported.

This can be fine when importing from Twitter, since we can't really
link a note to a tweet.

And, for the moment, it's acceptable when importing from *key, because
the export doesn't contain the instance URL, so we can't resolve ids
to remote notes.

It's less fine when importing from Mastodon / Pleroma / Akkoma,
because in those cases we _can_ link to the remote note that the user
was replying to.

This commit makes `recreateChain` optionally return "orphaned" note
trees, so in the (near) future we can use it to properly thread
imported notes from those services.