Management command to generate thumbnails that may be missing

Occasionally, you can create problems when syncing data from production environments or you just want to clear out unused thumbnails. I thought this seemed like a useful tool that others might need.
You can rewrite this to write the result directly to the file:
```
with open(ren.file.path, 'wb') as output
    ren.filter.process_image(ren.image.file, output)
```