Add categories to blog posts

[samrayner]

• List categories on Posts#index and Posts#show
• Filter Posts#index by clicking categories in list
• Add categories toolbar to blog post admin
• List categories in blog post index table rows
• Provide checkboxes for categories in admin post form
• Fix double render error on blog 404

[GBH]

Hooked up category filtering on the front-end. Part of the view that gets moved into the main app.