Implement thumbnail system for user images and metadata images

[Lingomat]

Currently we are resizing a file into a tiny thumbnail on the session view. For large files from a digital camera, the effect is that it takes some time to load.

1. Image uploads should be thumbnailed and both the original and the thumbnail pictures saved.
2. There should be a system to associate a thumbnail with an image and there should be easy methods to retrieve the thumbnail.
3. Images should offer click-to-zoom to view the full sized assets (and download).