Songs: Find covers of a Vocaloid original song from UtaiteDB

[riipah]

For example, for the song Senbonzakura, we want to be able to list all utaite versions of that song that are on UtaiteDB. In this case the song includes a reference link to UtaiteDB and the same entry has been added there, but that's not always the case (most importantly the reverse is not always true: if the original is an utaite song, we generally don't add it on VocaDB).

This should be an on-demand feature that requires user interaction: a link that says for example "check for alternate versions on UtaiteDB", which either populates the list of alternate versions or shows a popup.

Because of the different conventions on VocaDB and UtaiteDB we have to support two different ways. If the song includes a reference link from VocaDB to UtaiteDB, we simply get derived versions of the song from UtaiteDB using the ID of the referred song. If the original isn't on UtaiteDB we have to search by link URL: find all non-original songs from UtaiteDB that include a reference link to this entry on VocaDB.

In this case we simply load derived versions of the song with ID 1052 on UtaiteDB. If there was no reference link to UtaiteDB, we would search UtaiteDB for songs with a reference link to this entry.