Add 'self' to ApplicationController CSP

This fixes the welcome page (and presumably other internal rails pages)
giving a Content-Security-Policy error whenever rack-mini-profiler tries
to load – which is enabled by default for new projects

railties/lib/rails/application_controller.rb

@@ -7,8 +7,8 @@ class Rails::ApplicationController < ActionController::Base # :nodoc:
   before_action :disable_content_security_policy_nonce!
 
   content_security_policy do |policy|
-    policy.script_src :unsafe_inline
-    policy.style_src :unsafe_inline
+    policy.script_src :self, :unsafe_inline
+    policy.style_src :self, :unsafe_inline
   end
 
   private