UI to read the remaining space

Quota enforcement is nearly finished in #841 , but obviously, we'd like the user to be able to track the amount that they are using.

The easiest fix for this is to have a simple API exposure, but AFAIK, we can't use an ACL to restrict who can see the remaining quota, and I don't think we should expose it.

So, the better solution is to put it in the RDF along with the quota. That's more work, but it also means that the space left doesn't need to be computed real-time, so it is a solution that would scale better.

However, as there are many issues open, it needs a different prioritization, so I'm opening this issue for it.