Add documentation for running system specs on Ubuntu

[gravitystorm]

Our system specs (and our teaspoon specs) don't work on Ubuntu, due to incompatibilities with firefox snap. We should document that they don't work, or we should document how to make them work, or (ideally) figure out if there's a way to get them to work automatically.

The root of the problem is that on Ubuntu, firefox is distributed as a snap package, and so the path returned by which firefox is not actually a firefox binary, it's just a bit of snap-related code.

My personal not-quite-working-workarounds are, in test/application_system_test_case.rb:

 class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
   driven_by :selenium, :using => :headless_firefox do |options|
     options.add_preference("intl.accept_languages", "en")
+    options.binary = "/snap/firefox/current/usr/lib/firefox/firefox"
   end

I've still found problems with some javascript stuff that I haven't yet resolved.

For /test/teaspoon_env.rb:

   config.driver_options = {
     :client_driver => :firefox,
     :selenium_options => {
-      :options => Selenium::WebDriver::Firefox::Options.new(:args => ["-headless"])
+      :options => Selenium::WebDriver::Firefox::Options.new(:args => ["-headless"], :binary => "/snap/firefox/current/usr/lib/firefox/firefox")
     }
   }

I'm not sure whether documenting that they don't work is easier than documenting the workarounds!