Update value of User-Agent HTTP request header used by listers

That HTTP header value will now contain the lister name but also a link to our contact form in order for sysadmins to easily reach us if needed.

The following template is used to generate it:

"Software Heritage <lister_name> lister v (+https://www.softwareheritage.org/contact)"

Depnds on !311 (closed)

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Migrated from D8523 (view on Phabricator)