Use headless Chrome for browser tests

[vbrown608]

Loading the react and react-ujs packages requires loading ES6 syntax, which isn't supported by our current webdriver (capybara-webpack) or the one we're using on the Action Center (poltergeist). This PR switches SEC to use headless Chrome, which would allow us to write JS tests for the admin interface (ex #198) and add React to the user-facing site without breaking the existing JS tests.

This is working smoothly except within Docker, where there are two problems

1. The version of chromium-chromedriver packaged for Alpine is buggy - we need a newer version. It's probably possible to do a manual install instead of using the package.
2. Chrome/Chromium needs to make syscalls that are not allowed by the default Docker seccomp config. Jesse Frazelle has a blog about it here and links to a seccomp profile that can be used with Chrome/Chromium here. Seems like a lot of people are using it to run headless Chrome in Docker.

Options I see are

1. Update our current Dockerfile to manually install the chromedriver and update docker-compose.yml.example to suggest the Chrome seccomp outside production.
2. Try to use someone else's headless Chrome container and let selenium connect to Chrome over a port. That would keep some messiness out of the SEC application container.
3. Not support this in Docker, since I don't think any of us is doing development in Docker anymore.

[wioux]

I like option (2) and I think it'd keep techops happy as well. Ben and I discussed a related issue to this the other day, which was rendering PDFs from a headless chrome process running in a separate container. He was concerned about xvfb being included in the app container so I think if we add chromium too there'll be the same concern and we'll eventually need to pull it out.

The tests for supporters.eff.org work this way and we have a Dockerfile for chrome headless in kittens at tests/Dockerfile.google-chrome.

For problem (1) -- I'm not sure whether you tried it but when I was working on the PDF issue I used the chromedriver-helper gem to install chromedriver and that seemed to work well.

[vbrown608]

The chromdriver-helper gem is nice. I switched it in for tests run on the host and on Travis, thanks! It doesn't work within the Alpine container for the reason described here: flavorjones/chromedriver-helper#48 (comment)

I'd like to revisit Docker support as a separate ticket.