feat: privacy mode

Adds support for hiding IP addresses & hostnames associated with clients sending
authenticated mail in to Postal over SMTP and HTTP

db/migrate/20240206173036_add_privacy_mode_to_servers.rb

@@ -0,0 +1,5 @@
+class AddPrivacyModeToServers < ActiveRecord::Migration[6.1]
+  def change
+    add_column :servers, :privacy_mode, :boolean, default: false
+  end
+end