Convert to Selenium Webdriver

[michiels]

While kicking off some more development today, I noticed that Poltergeist is not supported/maintained anymore.

This PR converts the integration+browsers tests to a setup that is already aligned with new Rails 5.2 capabilities:

• Use selenium-webdriver and chromedriver-helper gems.
• Set headless chrome settings in test_helper.rb
• Set Puma to be used as Capybara server with single thread mode configured

[michiels]

Already done in #238