Add a route for viewing a submission on a browser

[ento]

Motivation: as someone on the reviewer side, I want to check out submissions even those I'm not assigned as the reviewer: to check out a particularly good solution, or a solution using a particular framework. However, downloading from S3 and opening the file involves too many clicks and keystrokes.

It'd be nice if the take-home server can serve up submissions.

For zip submissions: zip.js https://gildas-lormeau.github.io/zip.js/