Move away from capybara-webkit

[blerner]

capybara-webkit

Development has been suspended on this project because QtWebKit was deprecated in favor of QtWebEngine, which is not a suitable replacement for our purposes.

We instead recommend using the Selenium or Apparition drivers.

Apparently we already don't use capybara-webkit in Hourglass, but we also don't have as many UI test cases there... capybara-webkit relies on libqt4 (I think; maybe it'll accept qt5 instead) which isn't readily available on Ubuntu 20.04, so we ought to upgrade.

[blerner]

https://github.com/teamcapybara/capybara#selenium -- We might need to use the apparition driver, to get things like the screenshots to work.