Upgrade to Rails 5.2

All the code in the `bin/` and the `config/` folder has been generated
running `rake app:update`, except the `escape_javascript_fix` file,
which we've removed since the code there is already included in Rails
5.2.

bin/update

@@ -1,10 +1,9 @@
 #!/usr/bin/env ruby
-require "pathname"
 require "fileutils"
 include FileUtils
 
 # path to your application root.
-APP_ROOT = Pathname.new File.expand_path("../../", __FILE__)
+APP_ROOT = File.expand_path("..", __dir__)
 
 def system!(*args)
   system(*args) || abort("\n== Command #{args} failed ==")