feat: new configuration system (and schema) (#2819)

config/initializers/secret_key.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
-if Postal.config.rails&.secret_key
-  Rails.application.secrets.secret_key_base = Postal.config.rails.secret_key
+if Postal::Config.rails.secret_key
+  Rails.application.secrets.secret_key_base = Postal::Config.rails.secret_key
 else
   warn "No secret key was specified in the Postal config file. Using one for just this session"
   Rails.application.secrets.secret_key_base = SecureRandom.hex(128)