Upgrade Rails to version 5.2.1 and adopt code & specs where necessary

Signed-off-by: Sebastian Serth <Sebastian.Serth@student.hpi.de>

bin/yarn

@@ -0,0 +1,11 @@
+#!/usr/bin/env ruby
+APP_ROOT = File.expand_path('..', __dir__)
+Dir.chdir(APP_ROOT) do
+  begin
+    exec "yarnpkg", *ARGV
+  rescue Errno::ENOENT
+    $stderr.puts "Yarn executable was not detected in the system."
+    $stderr.puts "Download Yarn at https://yarnpkg.com/en/docs/install"
+    exit 1
+  end
+end