These had previously been forgotten, leading to NameErrors that
were caught and treated as if students had not been identified.

Fixes #115.

This makes it easier to reason about zejse when importing it from
a REPL.

os.rename apparently tries to do things with hardlinks.
Who knew?

The docs for react-dropzone say that we have to explicitly
free the 'file preview' to avoid memory leaks in the browser (!).
Presumably disabling previews means we don't have to manage this
ourselves.

The backend still does nothing

This, and using 'Object.assign', make it easier to add pieces
to the state afterwards without having to change *all* invocations
of 'setState'.