FEATURE: Add experimental option for strict-dynamic CSP (#25664)

The strict-dynamic CSP directive is supported in all our target browsers, and makes for a much simpler configuration. Instead of allowlisting paths, we use a per-request nonce to authorize `<script>` tags, and then those scripts are allowed to load additional scripts (or add additional inline scripts) without restriction.

This becomes especially useful when admins want to add external scripts like Google Tag Manager, or advertising scripts, which then go on to load a ton of other scripts.

All script tags introduced via themes will automatically have the nonce attribute applied, so it should be zero-effort for theme developers. Plugins *may* need some changes if they are inserting their own script tags.

This commit introduces a strict-dynamic-based CSP behind an experimental `content_security_policy_strict_dynamic` site setting.

lib/middleware/csp_script_nonce_injector.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module Middleware
-  class GtmScriptNonceInjector
+  class CspScriptNonceInjector
     def initialize(app, settings = {})
       @app = app
     end
@@ -9,10 +9,10 @@ module Middleware
     def call(env)
       status, headers, response = @app.call(env)
 
-      if nonce_placeholder = headers.delete("Discourse-GTM-Nonce-Placeholder")
-        nonce = SecureRandom.hex
+      if nonce_placeholder = headers.delete("Discourse-CSP-Nonce-Placeholder")
+        nonce = SecureRandom.alphanumeric(25)
         parts = []
-        response.each { |part| parts << part.to_s.sub(nonce_placeholder, nonce) }
+        response.each { |part| parts << part.to_s.gsub(nonce_placeholder, nonce) }
         %w[Content-Security-Policy Content-Security-Policy-Report-Only].each do |name|
           next if headers[name].blank?
           headers[name] = headers[name].sub("script-src ", "script-src 'nonce-#{nonce}' ")