Tools directory + supporting changes

:fork_and_knife: Alright here we go. This PR builds out the cookbooks index view and atom feed and refactors a bunch of stuff to support that feature. Here are the highlights. 

- Make the cookbook partial and simple cookbook partial styles more generic `listing` and `simple_listing` so they can be used for tools.
- Abstract the common UI element title with buttons into a class we can reuse throughout the application (and we are now).
- Remove a bit of dead SCSS.
- Limit tools so they can of the type `knife_plugin`, `ohai_plugin` or `chef_tool`.
- Add some tool seed data to work with in development.
- A few bug fixes
  - Seeds failed if you ran them twice, they don't anymore
  - Server side validations weren't working for the new tool form.
Given that this parameter has a finite set of valid values, I think it'd be worth to also check that it's a valid type. Thoughts?