Import header design and re-factor

and it didn't follow the designs from #94.

A few major changes:
- Switch to using as many bootstrap classes as possible
- Factor the header into three types:
  - application --> for authentication and all error/etc pages
  - marketing --> for the home page and regular docs
  - dashboard --> for our dashboard application
- Make fonts and spacing line up with wireframes, be more clean

The header is also split into partials to be more manageable.

Controllers can set their default layout in config/application.rb
to be different than application, which will render the other
headers accordingly.