Create thumbnail endpoint for public collections

[emma-sg]

Currently we return a presigned s3 url for thumbnails, which expires. For use cases such as the website (which pull thumbnails at build-time), a thumbnail url that doesn't change will prevent us needing to rebuild every week in order to prevent collection thumbnail images from erroring.

This should probably just be a redirect with a cache header set? I don't think we need to bother proxying the thumbnail — clients loading this url will just get a new url when necessary.