Add cypress e2e test to confirm PRs don't break app

[patcon]

Includes commits from #38

Cypress is an end-to-end testing tool. It can run in CI environments like GitHub Actions.

This PR adds:

• cypress boilerplate to minimally allow it to work
• a single e2e test for running through the form, including confirming validation works (it does not test file upload at the moment)
• a github action workflow to build the app, serve it from a one-off docker container, and run e2e test against it
• the workflow also saves the video captures that cypress makes for each test run, and preserves them as build artifacts on github (example at bottom of build page)

This PR also includes fixes from #37, so that the app works to test.

Hopefully this helps you manage PRs more easily :) Thanks again for this resource!