Add slugs to post URL's

For SEO, it's nice to have the post title in the URL.  This PR changes the canonical route for a post to be:

    /posts/:_id/p/:slug

The "/p/" part is so that a post with slug "edit" doesn't conflict with ``/posts/:_id/edit``.  Routes to ``/posts/:_id`` redirect to include the slug; though the index pages at least are updated to point directly to the slugified versions.