Fixes for comments to non-existing subject pages

* Disallow posting comments to normal non-existing pages
* Allow posting comments to non-existing user talk pages of registered users (and auto-create the subject page)
* Always notify the user about the comments to his talk page
Fetch user_email at once in comment notifications, without additional DB queries,
and do not send them to non-authenticated emails.
Check email authentication in new post emails, make code more compact
Include new post title in notification emails
Use user language in new post notifications