(Bug 5073) Update us to Foundation 5

Only customizations are in foundation/_settings.scss and in
foundation/foundation.scss
(Bug 5073) Set the lightness of the callout panel based on panel

Instead of lightening by an arbitrary 60% (doesn't work on most skins:
it ends up being either white, e.g., Tropo Purple, or goes the wrong
way, e.g., Gradation)
(Bug 5073) Add border back to the buttons
(Bug 5073) Use $input-prefix-border-color
(Bug 5073) Set body base line height to $base-line-height
(Bug 5073) Fix topbar in Celerity, Gradation-*

Also adds an example of topbar in /dev/style-guide
Merge pull request #607 from afuna/foundation-5-bug5073

(Bug 5073) Update to Foundation 5