External template for email notifications

[wavexx]

The current email notifications are generated by functions in include/msg.php. This plays "somewhat" nicely with localization, but it's tedious to maintain and cannot be easily customized.

We should move the templates to something external, with a minimal template engine.
Smarty comes to mind. It would allow to have a mostly plain template, with "if" blocks for variable text (such as password/comment info).