LatestActivity: set caching TTL in app

Instead of VCL

```c
    if (req.url ~ "LatestActivity") {
        set beresp.ttl = 600s;
        set beresp.grace = 60s;
        set beresp.http.X-Cacheable = "YES - FORCED";
        return(deliver);
    }
```

@michalroszka / @prein