expose total storage usage on Usage quotas page

With backend implementation available on staging and UX already defined in gitlab-org/gitlab-ce#59232, we can move on with frontend development.

Whats need to be done

1. the GraphQL query should be expanded to fetch rootStorageStatistics

   {
     namespace(fullPath: "a_namespace_path") {
       id
       rootStorageStatistics {
         storageSize
       }
       projects(includeSubgroups: true) {  /* ... */ }
     }
   }

2. rootStorageStatistics.storageSize should be exposed as Total usage on Usage quotas page inside the storage tab

On GDK I got nil for rootStorageStatistics

Storage statistics get updated only with an event that affects the value. It may be nil, but to force a value on gdk it will be enough to push a commit (or edit a file with the web interface)

Please refer to gitlab-org/gitlab-ce#59232 and Removing the storage limit paragraph on the same issue for additional details.