Add script to save static pages after JS has run

#15639

We want to be able to generate some pages as part of the static site
build after JavaScript has run on that page. This will allow these pages
to be populated with data from JavaScript but appear to progressively
enhance when deployed as part of our static site.

This script will fetch a list of URLs from an endpoint on the
application that should be scraped with JS enabled. It will then use
capybara-webkit to fetch those pages, run JS, and save those pages to
disk. The target files will be in the same location as `wget` uses which
will allow us to overwrite the static files already generated by `wget`
with these JS-enhanced versions.

Subsequent commits will need to:

* Create the endpoint that this script uses to determine which URLs to
  scrape
* Update the build script to run this after `wget` has run

.travis.yml

@@ -9,7 +9,7 @@ addons:
     - cmake
 script:
   - xvfb-run bundle exec rake
-  - travis_wait 30 scripts/release.sh
+  - travis_wait 30 xvfb-run scripts/release.sh
   - bash <(curl -fsSL https://github.com/everypolitician/ensure-regression-tests/raw/master/ensure-regression-tests)
 sudo: false
 rvm:

scripts/save_javascript_pages.rb

@@ -0,0 +1,70 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require 'capybara-webkit'
+require 'fileutils'
+require 'open-uri'
+
+class PageAfterAJAX
+  attr_reader :page, :url
+
+  def initialize
+    Capybara::Webkit.configure(&:allow_unknown_urls)
+    @page = Capybara::Session.new(:webkit)
+  end
+
+  def save(url)
+    @url = url
+    visit_and_wait
+    restore_pre_js_page_classes
+    write_page_to_disk
+    puts "Saved #{filename}"
+  end
+
+  private
+
+  def write_page_to_disk
+    create_parent_directories
+    File.write(filename, page.body)
+  end
+
+  def visit_and_wait
+    page.visit(url)
+    wait_for_ajax
+  end
+
+  # Restores page classes modified by running JS on the page
+  def restore_pre_js_page_classes
+    page.execute_script("$('html').addClass('no-js')")
+    page.execute_script("$('html').removeClass('flexwrap')")
+  end
+
+  def create_parent_directories
+    FileUtils.mkdir_p(File.dirname(filename))
+  end
+
+  def filename
+    url_path[-5..-1] == '.html' ? url_path : "#{url_path}.html"
+  end
+
+  def url_path
+    URI.parse(url).path[1..-1]
+  end
+
+  def wait_for_ajax
+    Timeout.timeout(Capybara.default_max_wait_time) do
+      loop until finished_all_ajax_requests?
+    end
+  end
+
+  def finished_all_ajax_requests?
+    page.evaluate_script('jQuery.active').zero?
+  end
+end
+
+page = PageAfterAJAX.new
+javascript_pages_to_scrape = open(ARGV[0]).read.split
+
+javascript_pages_to_scrape.each do |url|
+  page.save(url)
+end