List blogs by URL user rather than request user

List blogs by URL user rather than request user

The blog_post_listing function in mediagoblin/media_types/blow/views.py
attempts to access blogs based on the requesting user rather than the url
user. This results in server errors when an unauthenticated user attempts to
follow a link from another user's blog post listing, and 404 errors when an
authenticated user attempts to follow a link from another user's blog post
listing.

This change bases blog post listings on the URL user rather than the request
user.