Update AR::Tenanted to have tenanted blob keys

Also, a script to migrate blobs (on disk and in the database) to the
new convention.

ref: https://37s.fizzy.37signals.com/collections/7/cards/246

script/migrate-disk-service-blobs.rb

@@ -0,0 +1,28 @@
+#! /usr/bin/env ruby
+
+require_relative "../config/environment"
+
+ApplicationRecord.with_each_tenant do |tenant|
+  puts "\n## #{tenant}"
+  report = { updated: 0, skipped: 0 }
+
+  ActiveStorage::Blob.find_each do |blob|
+    if blob.key.start_with?("#{tenant}/")
+      report[:skipped] += 1
+    else
+      blob.update_column :key, "#{tenant}/#{blob.key}"
+      report[:updated] += 1
+    end
+  end
+  pp report
+
+  disk_service = ActiveStorage::Blob.services.fetch(:local)
+  new_root = File.join(disk_service.root, tenant)
+  old_root = File.join("storage", "tenants", Rails.env, tenant, "files")
+
+  FileUtils.mkdir_p(new_root, verbose: true) unless File.directory?(new_root)
+
+  Dir.glob(File.join(old_root, "??")).each_slice(20) do |blob_dirs|
+    FileUtils.mv(blob_dirs, new_root, verbose: true)
+  end
+end