FS blobs: implement a basic quota

As part of the FSBlobsBackend, I added a get_size_for(user) to the api. My thinking is that with the fs is very easy to get the total storage size reported from the FS, and thus avoid a very easy DoS case.

• get the max quota hardcoded in the class. something like 200MB to begin with?
• when writing the blob, raise a QuotaExceeded if limit reached!