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

app/views/app_mailer/server_suspended.text.erb

@@ -8,5 +8,5 @@ Reason: <%= @server.actual_suspension_reason %>
 
 Thanks,
 
-<%= Postal.smtp_from_name %>
-<%= Postal.smtp_from_address %>
+<%= Postal::Config.smtp.from_name %>
+<%= Postal::Config.smtp.from_address %>