DEV: Introduce Migration::Helpers for new-site detection (#20934)

We use schema_migration_details to determine the age of a site in multiple migrations. This commit moves the logic into a dedicated `Migration::Helpers` module so that it doesn't need to be re-implemented every time.

lib/migration/helpers.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+module Migration
+  module Helpers
+    def self.site_created_at
+      result = DB.query_single <<~SQL
+        SELECT created_at
+        FROM schema_migration_details
+        ORDER BY created_at
+        LIMIT 1
+      SQL
+      result.first
+    end
+
+    def self.existing_site?
+      site_created_at < 1.hour.ago
+    end
+
+    def self.new_site?
+      !old_site?
+    end
+  end
+end