citations

Publications typically have a cluster ID that identifies that multiple versions of the same article. We need to scrape it and add it to a filled Publication entry object. Motivated by #350 .

Example: https://scholar.google.com/scholar?cluster=2680296923030532700&hl=en&as_sdt=0,31

I imagine we don't actually want people to add references to user pages and their subpages, but on some Wikipedias there are quite a lot of {{cn}} in those. It seems to be the case for Catalan, for example:

https://citationhunt.toolforge.org/ca?id=340f6a16
https://citationhunt.toolforge.org/ca?id=227467f3

I think we can easily identify those pages because their [page_namespace](https://www.