Resolve "Use nodeinfo schema for instance statistics"

Closes #192 (closed)

Feedback welcome!

/.well-known/nodeinfo payload:

{
  "links": [
    {
      "rel": "http://nodeinfo.diaspora.software/ns/schema/2.0",
      "href": "https://node1.funkwhale.test/api/v1/instance/nodeinfo/2.0/"
    }
  ]
}

/api/v1/instance/nodeinfo/2.0/ payload:

{
    "version": "2.0",
    "software": {
        "name": "funkwhale",
        "version": "0.11"
    },
    "protocols": [
        "activitypub"
    ],
    "services": {
        "inbound": [],
        "outbound": []
    },
    "openRegistrations": true,
    "usage": {
        "users": {
            "total": 6
        },
        "localPosts": 0,
        "localComments": 0
    },
    "metadata": {
        "shortDescription": "The best instance out there",
        "longDescription": "Lorem ipsum dolor sic amet",
        "name": "Funk the system",
        "library": {
            "federationEnabled": true,
            "federationNeedsApproval": true,
            "tracks": {
                "total": 2327
            },
            "artists": {
                "total": 108
            },
            "albums": {
                "total": 192
            },
            "music": {
                "hours": 1.9491666666666667
            }
        },
        "usage": {
            "favorites": {
                "tracks": {
                    "total": 1
                }
            },
            "listenings": {
                "total": 8
            }
        }
    }
}