hide all dangerous folders under a single dangerzone/ folder

Right now we have dangerzene-processed/, dangerzone-rejected/, dangerzone-processing/... that's noisy, at best, and dangerous, at worst: at least one user thought that dangerzone-processed was where processed (so: safe) files ended up.

Just have dangerzone/ and safe/ folders at the toplevel, with the processed/, rejected/, processing/ stuff inside dangerzone/.

Closes: #4 (closed)