Don't use referrer as a fallback in VDestination.

Since the referrer is not included in cache keys for pagecache or
Akamai, this can result in weird stuff getting cached and causing
downstream users to get redirected after login/registration to
unexpected destinations.
Use correct fullname prefixes for message parents

Previously, all parent_id values would be prefixed with t4_ (message),
even if the message was from a comment, which would actually have either
a comment or link parent.
Adjust numbered lists margin to show two digits.