Create thumbnails on photo upload

[smashnet]

For each uploaded photo, create a large-, mid- and small-sized thumbnail.

Small: 128px width
Mid: 512px width
Large: 1024px width

Thumbs should be located in data/img/thumbs/ and follow the scheme {uuid}_{size}px.{fileext}

[smashnet]

Possible future improvement:
Generate thumbs asynchronously so that the POST request can be answered quickly.