Set up testing

We should have some rudimentary validation of the web frontend for changes on Nemo, so that we can catch issues like #14 and #15 early. Automatically deploying a bleeding-edge version would also be nice.

This is related to knowsys/nemo#618 (and, more generally, knowsys/nemo#616), but could probably also be implemented by mocking relevant parts of nemo-wasm.