DEV: Memoize CSP nonce placeholder on response (discourse#25724)

That way, the same value is used even if the helper is called in the context of different controllers

Followup to c8a1b49

app/helpers/application_helper.rb

@@ -66,12 +66,9 @@ def google_tag_manager_json
   end
 
   def csp_nonce_placeholder
-    @csp_nonce_placeholder ||=
-      begin
-        placeholder = "[[csp_nonce_placeholder_#{SecureRandom.hex}]]"
-        response.headers["Discourse-CSP-Nonce-Placeholder"] = placeholder
-        placeholder
-      end
+    response.headers[
+      ::Middleware::CspScriptNonceInjector::PLACEHOLDER_HEADER
+    ] ||= "[[csp_nonce_placeholder_#{SecureRandom.hex}]]"
   end
 
   def shared_session_key

lib/middleware/csp_script_nonce_injector.rb

@@ -2,14 +2,16 @@
 
 module Middleware
   class CspScriptNonceInjector
+    PLACEHOLDER_HEADER = "Discourse-CSP-Nonce-Placeholder"
+
     def initialize(app, settings = {})
       @app = app
     end
 
     def call(env)
       status, headers, response = @app.call(env)
 
-      if nonce_placeholder = headers.delete("Discourse-CSP-Nonce-Placeholder")
+      if nonce_placeholder = headers.delete(PLACEHOLDER_HEADER)
         nonce = SecureRandom.alphanumeric(25)
         parts = []
         response.each { |part| parts << part.to_s.gsub(nonce_placeholder, nonce) }