Enforce a script policy that allows the importmap by hash

script-src 'self', object-src 'none' and base-uri 'self', enforced. The
two inline scripts the importmap helper emits are allowed by their
SHA-256, and the origins its URLs point to (a configured asset host, in
any form Rails accepts) are allowed by name. There is no per-request
nonce, so cached pages stay valid.

DISABLE_CONTENT_SECURITY_POLICY=true leaves the policy off, for an
install whose proxy or customization injects scripts.

(cherry picked from commit 562888251e)

Backport: 1.5.2 has no response cache, so the cached-page test is left out.

config/initializers/content_security_policy.rb

@@ -1,25 +1,18 @@
-# Be sure to restart your server when you modify this file.
-
-# Define an application-wide content security policy.
-# See the Securing Rails Applications Guide for more information:
-# https://guides.rubyonrails.org/security.html#content-security-policy-header
-
-# Rails.application.configure do
-#   config.content_security_policy do |policy|
-#     policy.default_src :self, :https
-#     policy.font_src    :self, :https, :data
-#     policy.img_src     :self, :https, :data
-#     policy.object_src  :none
-#     policy.script_src  :self, :https
-#     policy.style_src   :self, :https
-#     # Specify URI for violation reports
-#     # policy.report_uri "/csp-violation-report-endpoint"
-#   end
+# Enforcing baseline policy. Scripts run only from this origin; the two inline
+# scripts the importmap helper emits are allowed by hash (InlineScriptHashes),
+# so there is no per-request nonce and rendered HTML stays cacheable.
 #
-#   # Generate session nonces for permitted importmap and inline scripts
-#   config.content_security_policy_nonce_generator = ->(request) { request.session.id.to_s }
-#   config.content_security_policy_nonce_directives = %w(script-src)
+# Nothing else is restricted: images, media, fonts, styles (including an
+# account's custom styles) and connections behave as before.
 #
-#   # Report violations without enforcing the policy.
-#   # config.content_security_policy_report_only = true
-# end
+# DISABLE_CONTENT_SECURITY_POLICY=true is the escape hatch for an install whose
+# proxy or customization injects scripts the policy would block.
+unless ENV["DISABLE_CONTENT_SECURITY_POLICY"] == "true"
+  Rails.application.configure do
+    config.content_security_policy do |policy|
+      policy.script_src :self
+      policy.object_src :none
+      policy.base_uri   :self
+    end
+  end
+end