Fetch Wikidata ID if Wikipedia tag used

[pigsonthewing]

Wikipedia plugin:

If a user enters a Wikipedia tag, wikipedia=xx:foo (where "xx" is a two-letter ISO language code, and "foo" is the tile of a Wikipedia article), lookup (via the Wikidata API) and include the related Wikidata item.

For example, for "wikipedia=en:St James' Church, Handsworth", the Wikidata item's ID is "Q16908818"; see [1], [2].

Show a validator warning if no Wikidata item is found (the Wikidata item may not exist yet; or the value entered may contain an error.

Show a validator warning if two or more Wikipedia articles are linked, with different Wikidata item IDs (may indicate an error; or there may be duplicates on Wikidata).

[1] ​https://en.wikipedia.org/wiki/St_James%27_Church,_Handsworth

[2] ​https://www.wikidata.org/wiki/Q16908818

[anonymous]

Wikipedia and Wikidata don't have a 1:1 relation.

[simon04]

This query can be used: ​https://www.wikidata.org/w/api.php?action=wbgetentities&sites=enwiki&titles=London|Vienna&props=labels

[simon04]

Fetching implemented in [o31852:31854]

Validation → #12219