offline: show the offline mode and open breakers in the stats API and on the setup page

The offline mode and the per-upstream breaker state (#57 (closed), !35 (merged)) can only be read from the admin socket (GET /v1/offline, pkgproxyctl offline status). Someone watching a dashboard through the stats API, or a remote visitor on the setup page, cannot tell that pkgproxy is refusing to contact some upstreams.

Add the mode and the open upstreams, with their retry times, to the stats socket's GET /v1/info. The stats socket is read-only and safe to leave broadly reachable, unlike the admin socket. Also show a notice on the setup page. Document the new fields in docs/statsapi.md (see #60 (closed)).

Follows up #57 (closed).