support thumbnailing

[graft]

Many files on metis will be images; metis should support thumbnailing of these images in some way, probably via a worker process. Thumbnails might work like this:

1. When an upload is made/a file exists, a "thumbnail" may be requested for that file at a given location (the name of the thumbnail is up to the requester).

2. Optionally Metis puts a placeholder in place for the thumbnail.

3. Metis builds a thumbnail and puts it in place at the requested location.

Questions:

Where is the work queue maintained? How does Metis check for new work?