Modernized the app with app structure from Stage Manager

Rakefile

@@ -1,11 +1,17 @@
 require 'bundler/setup'
 
 namespace :server do
-  task :dev do
-    %x{puma -C puma.rb}
+  task :start do
+    system('bundle exec puma -C config/puma.rb')
   end
 
   task :reload do
-    %x{rerun --no-notify 'puma -C puma.rb'}
+    system('guard')
+  end
+end
+
+namespace :test do
+  task :lint do
+    system("rubocop app/ server.rb")
   end
 end