It turns out that 'site' is a standard module, and if you are using
pytest and try to `import site` python gets confused about which `site`
you mean.  We could untangle that via some sys.path hacking or import
module munging, or maybe just symlinks, but this is cleanest.

 * Removed Uncategorized as a category
 * Made 'Posts' the default category
 * Removed mention of Posts everywhere except the 'categories' page
 * Corrected post that had multiple categories (max is 1)
 * Retagged and categorized posts
 * Added categories to sidebar