Let users merge duplicate entries

[pneutig]

Is your feature request related to a problem? Please describe.
Frequently you will find duplicate entries of the same book. There is no apparent way as a user to contribute to fixing duplicate entries. All I found for that was for administrators of the instance.

Describe the solution you'd like
Instance users will typically be the ones to discover duplicate entries. Ideally users can contribute themselves to fixing that problem. Suggestion: add a button either on the page of the book or within the edit book form, titled "Merge duplicate book entries". Users can then enter either the number of the book entry on their instance or a link to it. Where data of the two entries are different, offer to either add them as separate editions or to prioritise the data based on one of the entries. Bonus: let users choose per individual data part, which one to prioritise or even edit them during the merging process. The old entry will then link to the merged entry, so that reviews etc. will still lead to the correct book and show up on its page. Bonus: send notification to users who wrote a review for an entry that was merged, to let them remove their review if it has been maliciously / falsely merged to a different book than what they had reviewed.

Describe alternatives you've considered
Provide a guide for users to manually edit duplicate entries in a way that will merge them.
Provide a button to notify the administrator of an instance about duplicate entries, which allows to report multiple entries at once.