Make uploads faster by generating and saving thumbnails in parallel.
We generate each thumbnail in parallel, then send each thumbnail to the
backend image servers in parallel.
Most images have 5 variants: 'preview' (150x150), 180x180, 360x360,
720x720, and 'sample' (850px width). Plus the original file, that's 6
files we have to save. In production we have 2 image servers, so we have
to save each file twice, to 2 remote servers. Doing all this in parallel
should make uploads significantly faster.
Fix uploads failing in the default configuration because the backup
storage manager didn't initialize the null storage manager with the
required `base_url` and `base_dir` options.
