Make the source of the object counts configurable

The webapp retrieves the count of objects from storage : https://forge.softwareheritage.org/source/swh-web/browse/master/swh/web/common/archive.py$920-926

In order to progressively deploy in staging and production, it should be made configurable to get them from storage or swh-counters

---

Migrated from T3231 (view on Phabricator)