UI Update for Project and Document pages

* New very stylish buttons for edit/new/join in project/document pages
* Replaced table in Project index page with an unordered list
* Removed sidebar affix behaviour
* Updated `seed.rb` and `regenerate_slugs.rake`
* Added breadcrumb navigation to Project and Document show page
* New file `_mixins.scss` containing some CSS functions
* Updated cukes and specs to reflect new UI