Link Wikidata entities (Qxxx) to SWH origins with a new property/qualifier

Goals:

To improve the visibility of SWH on WikiData
adding links from WikiData to SWH

Options:

SWH origin add a property as an external identifier
In swh/devel/swh-web#1346 using archive urlas a qualifier of the repository property (not very visible)
SWH archive url property

This is different from the SWHID which is used on different classes of items

Migrated from T2396 (view on Phabricator)

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