Improve layout of header and menu
Make menu more consistent
Rename query method and update model

* Rename the query method to make more sense in the context of calling
  the query.
* Make the blog post model's slug field unique across all posts,
  instead of only across the day of the post.
Add previous and next links

In each post, link to the previous and next post, ordered by publish
date.

Also, unify the usage of self.kwargs to get the url kwargs, instead
of misunderstanding the intent of the get_context_args **kwargs.