Cleanup styling and DOM structure of page layout

* Add bars between navigation links.
* Change <nav> to a group of <a>s
* Nest <nav> inside <header>
* Use classes to style <header> and <nav>
Extract 'actionable' class for easier reuse
More descriptive nav links
Fix specs from changes to layout markup
Cleanup listing of receipts

* Add reusable `.table-striped` style
* Reduce padding on table cells
* Cleanup decorators, focusing them on formatting/presentation of data
  over generating markup
* Customized kaminari views
Update button styles
Tweak CSS for role management page

Display form on top and table below instead of side-by-side
Hide autocomplete input until role is selected