Add categories to posts

https://trello.com/c/XZllJLO5
Fix broken /admin/people/new
Add employee flag to people table
Don't display broken image if none is available

https://trello.com/c/LygpDMgm
Merge branch 'guest-blog-posts'

* guest-blog-posts:
  Don't display broken image if none is available
  Add employee flag to people table
Move alignment class out of blog
First pass through on new style for /blog
Design tweaks for mobile
Separate post and post-preview style
Reposition footer