web API: allow to override file names upon download

When downloading content blobs via the API (e.g, from this URL), the download file name proposed by the browser is horrible, which is expected in the general case. However, in specific cases people might want to link to Software Heritage download URLs, specifying themselves the desired file name. We need to add the ability to specify the desired download file name when linking to /raw/ download URLs. As a first approximation, this might take the form of a ?filename=... GET parameter.

---

Migrated from T695 (view on Phabricator)