Replace PhantomJS with Headless Chrome for GitLab Integration Tests

This is a transition from Capybara's Poltergeist/PhantomJS driver to Selenium WebDriver /w Headless Chrome. Because of differences in driver support and behavior, and the transition to a more accurate and capable browser, this introduces several test failures that needed to be addressed.

To Test Locally:

1. Install Google Chrome and the latest version of ChromeDriver (currently 2.33):
   https://sites.google.com/a/chromium.org/chromedriver/

2. Run bundle install

3. Run RSpec or Spinach tests as normal

Notes:

• You can disable headless mode by using export CHROME_HEADLESS=false in your terminal before running RSpec and watch the browser in real time as tests are run.

Tasks:

• Add Selenium and configure it within Capybara for RSpec and Spinach
• Remove Poltergeist and PhantomJS dependencies
• Await bug fix within ChromeDriver for send_keys (bug report - fixed in v2.31)
• Create work-around for missing request/response introspection features (status_code, response_headers, network_traffic)
• Create helper to work around missing support for send_keys with non-BMP characters. (bug report)
• Create helper to work around the more restrictive add_cookie implementation
• Fix tests where accept_confirm is needed to explicitly resolve modal javascript confirm dialogs
• Fix instances of trigger('click'); which were used to work-around poor PhantomJS behavior
• Fix tests which relied on hidden element behavior within PhantomJS
• Get all remaining tests to pass
• Create EE port and get all tests to pass there (gitlab-ee!3245)

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Older Notes:

Errors:

• unknown error: an X display is required for keycode conversions, consider using Xvfb (e.g. https://gitlab.com/gitlab-org/gitlab-ce/builds/18894684).

  Bug report: https://bugs.chromium.org/p/chromedriver/issues/detail?id=1772. Some users mention compiling with use_ozone on Chrome and ChromeDriver solves this problem.

  Alternatives: Use xvfb (https://github.com/Automattic/wp-e2e-tests/issues/515#issuecomment-301165065)

• https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/27344249: Selenium does not support status_code: https://stackoverflow.com/questions/7908907/how-to-test-the-response-code-with-capybara-selenium

• https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/28861088: Selenium does not support response_headers

• https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/28877991: Selenium does not support network_traffic: https://stackoverflow.com/questions/12034013/is-there-any-way-to-log-http-requests-responses-using-selenium-webdriver-firefo/12036058#12036058

Be sure to read https://makandracards.com/makandra/7617-change-how-capybara-sees-or-ignores-hidden-elements about how Chrome/Selenium behaves differently with hidden elements.

Closes #30876 (closed)