Complete the minimal filesystem backend for encrypted blobs on the server side.

Missing:

• discuss if there's something missing in the pluggable storage interface (over mail/mumble on may 11, not an issue)
• document the status of the filesystem storage backend #8850 (closed)
• make blobs resource backend configurable #8804 (closed)
• sanitize path in FilesystemBackend #8800 (closed)
• delete incomplete uploads #8802 (moved to 0.9.7)
• Ensure security on Quota measurement #8832 (closed)