Add UI for merging different works into editions of the same work

[WesleyAC]

As described in #562, there should be a UI to merge multiple different works into the same work as different editions:

I think the highest value is letting users manually merge works -- the script does more more quickly, but it's more important that people have editorial agency and the ability to fix the problems that bug them, and there's a lot you can't automate.

Pulling this out into a separate issue to track it more easily.

Probably on the "Editions" page (example) we should have a second button next to "Add another edition" to "Merge existing book as edition", which would let you search for a existing book (or maybe paste a URL would be a nicer UI?) to merge.

I'm not sure how the ID of the previous work should be handled, but presumably the existing merge script handles that somehow?

[mouse-reeve]

Right now, the "non-canonical" entry is simply deleted (this does not federate) and all foreign keys associated with it are relocated to the canonical entry

[hughrun]

The "proper" way to do this probably would be via an ActivityPub Move activity. This would potentially allow for a setting as per #2841 at each instance to control behaviour when a Move activity is received for a book - i.e. does it auto-merge, or does it go into a queue to be approved, is it dropped etc.

[WesleyAC]

@mouse-reeve Are existing URLs maintained? For instance, commenting on a book has a link to a book page on non-bookwyrm-compatible fedi services, does that go to the correct place after a merge? I guess maybe part of what I'm asking is, are the "book IDs" that I see in URLs IDs of editions, or IDs of works? Or a mix of both?

[mouse-reeve]

No, the urls aren't maintained, it just becomes a 404 -- not ideal! The IDs in urls are usually the edition ID, but if you enter the work ID, it will display the page for the default edition

[Minnozz]

I would love to contribute to adding this feature. Do you have any suggestions for where to start?