i18n messages in the scaffolding

The scaffolding used to include a messages folder for i18n messages. However, since this is a commonly unused feature, it was removed. To add back this feature, do the following:

1. In Foundation.hs, add the line mkMessage "App" "messages" "en". Replace "en" with your default language.
2. Create a directory "messages".
3. Create a file "messages/en.msg" with the following dummy content: Hello: Hello.

Then it will be possible to refer to the message in Hamlet templates with _{MsgHello}.

You can find comprehensive overview of messages in "Internationalization" article on Yesod web site.