Add Selenium test that validates:
* Viewing the intake form while logged out
* The form loads for logged in users
* Shows errors when submitting an incomplete form
* Reveals all projects when "All projects" checkbox is checked
* Hides errors on submission if all required fields are filled in
* Shows a confirmation message post-submission
* The page title is what it is expected to be
* The various fields have the expected values

For now, this isn't being ran in CI, and hence is not part of the normal
`npm test` task.

Other changes:
* New credentials system for Selenium tests if `MW_` env vars aren't set
* Update README with instructions on running the Selenium tests
* Bump node dependencies
* Add CSS classes to some elements (made it easier to test)