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.

app/controllers/application_controller.rb

@@ -1,4 +1,4 @@
 class ApplicationController < ActionController::Base
-  include AllowBrowser, Authentication, Authorization, BlockBannedRequests, SetCurrentRequest, SetPlatform, TrackedRoomVisit, VersionHeaders
+  include AllowBrowser, Authentication, Authorization, BlockBannedRequests, SetCurrentRequest, SetPlatform, TrackedRoomVisit, VersionHeaders, InlineScriptHashes
   include Turbo::Streams::Broadcasts, Turbo::Streams::StreamName
 end