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