When a user adds a Wikipedia URL, we should check whether there is a Wikidata page and if so, try to add it.

I'm not sure how best it should work, I can think of a few options:

• Use Javascript to query Wikidata and add the URL to the external links before submitting the form. Users would be able to remove the URL before submitting (which we don't really want them to), it would be submitted under the user's account (which is slightly weird) and there's the possibility that the URL won't get loaded in time. This also doesn't work for things like editing via the relationship editor endpoint.
• Add the URL immediately after submitting the Wikipedia URL as the current user. This would be weird because it would create edits which the user didn't realise they were creating. It might however work if an edit note is added saying where the URL came from.
• Add the URL immediately after submitting the Wikipedia URL as ModBot (like set track length edits after adding a disc ID). This might work, except if the Wikipedia URL gets voted down, the Wikidata URL should be removed too, so it would be weird to do it as ModBot.
• Run a script periodically to add Wikidata URLs as ModBot (where the Wikipedia URLs have no pending edits). This would work, but means there's a delay of up to a week before the Wikidata URL will get added. This is also essentially what murdos_bot is already doing.

For the first three options, there's also the question of whether we want to add the Wikidata URL as well as or instead of the Wikipedia URL. With STYLE-488 in mind, it should eventually be instead of the Wikipedia URLs, but I'm not sure if we want to implement it that way from the beginning.