Provide a BlobsController for stable blob URLs

We need to have stable urls for blobs and variants or caching won't work. So provide a controller that can give that and redirect to the service URL upon lookup.

app/controllers/active_storage/blobs_controller.rb

+# Take a signed permanent reference for a blob and turn it into an expiring service URL for its download.
+# Note: These URLs are publicly accessible. If you need to enforce access protection beyond the
+# security-through-obscurity factor of the signed blob references, you'll need to implement your own
+# authenticated redirection controller.
+class ActiveStorage::BlobsController < ActionController::Base
+  def show
+    if blob = find_signed_blob
+      redirect_to blob.url(disposition: disposition_param)
+    else
+      head :not_found
+    end
+  end
+
+  private
+    def find_signed_blob
+      ActiveStorage::Blob.find_signed(params[:signed_id])
+    end
+
+    def disposition_param
+      params[:disposition].presence_in(%w( inline attachment )) || "inline"
+    end
+end

config/routes.rb

 Rails.application.routes.draw do
+  get  "/rails/active_storage/blobs/:signed_id/*filename" => "active_storage/blobs#show", as: :rails_service_blob
+
+  direct :rails_blob do |blob|
+    route_for(:rails_service_blob, blob.signed_id, blob.filename)
+  end
+
+  resolve("ActiveStorage::Blob")       { |blob| route_for(:rails_blob, blob) }
+  resolve("ActiveStorage::Attachment") { |attachment| route_for(:rails_blob, attachment.blob) }
 
 
   get  "/rails/active_storage/variants/:signed_blob_id/:variation_key/*filename" => "active_storage/variants#show", as: :rails_blob_variation