Automatically add/update language links for languages hosted by external wikis

[lahwaacz]

https://wiki.archlinux.org/index.php/ArchWiki_talk:Requests#Automatically_add%2Fupdate_language_links_for_languages_hosted_by_external_wikis

Basically we would need to:

• read the external wiki's interwiki table, figure out what is the "English" wiki prefix
• when checking if there is a translation for the external language, find out if there is a unique page which has an interwiki link to the English page (we probably need some preprocessing for this...)