Localize a sentence in 403.html

I don't like this approach because it breaks a sentence into several
unrelated strings for translation, but can't come up with something
better now.
Split Commenter fields off of Comment struct

And start using it in a few places, more will follow.
Use text/template instead of printf

Construct email notification with text/template.
Use Commenter struct in DB funcs
Don't add protocol prefix to an empty string
Extract func to get host name from request
Fix URL in email notification