Add link from the main archive to the Bitbucket mercurial case (https://bitbucket-archive.softwareheritage.org/)

It is very hard to find a way to access bitbucket archived code if you don't know the address (https://bitbucket-archive.softwareheritage.org/).

One proposition is to link the icon on archive.softwareheritage.org to the archived content, but if we do so we need to follow the same pattern with other forges. First, is that possible? that the click will run a query with the forge url?

Another reason to do so, for example Gitorious has a broken link, from this point of view it is more effective to link to our Gitorious collection than to the external url.

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Migrated from T2640 (view on Phabricator)