Add simple thumbnail generation middleware.

[baamenabar]

a simple express middleware script to generate thumbnails

[ ] Triggered when image is uploaded...
[ ] ... or if not available when image info is requested, through the API.
[ ] thumbnails will be generated asynchronously
[ ] thumbnails can be one configurable size only (constrained within a square)
[ ] thumbnails will be stored in one folder next to the uploads folder, served via static
[ ] the middleware is activated via env config, on by default.
[ ] thumbnail path will be served with image info throuh the API.