Fix issues regarding templates

[bertoost]

1. Changing the load custom template method to use Craft's frontend template path. Extends and includes could (and should) be relative to the default templates folder. This is how Craft is working by default too.
2. Secondly changed the new confirmation email template setting. From the previous version, you could have set up a notification email template, which now becomes the only mail to the admin-user. The mail to the client becomes default again.
   When the new confirmation email template setting is not set, use the notification email template instead. Now backwards compatible.