(Bug 5073) Tweaks, bugfixes, refactoring

* header-nav mixin has changed name and arguments. Also does a few more
  things, so remove redundant code

* bugfix: "underline" => underline. Strings with quotes around them are
  used as-is. So our CSS ends up having:

     text-decoration: "underline";

  Yuck.
(Bug 5073) Rename header-nav mixin to main-nav

Said mixin includes some code for vertical layouts as well
(Bug 5073) Add skiplinks to foundation in tropo

* use the skiplink components (tt block template, .scss file)
(Bug 5073) Tweak alignment for nav search

* aligns the search form elements with one another
* aligns the search form to match the rest of the top bar
(Bug 5073) Shrinks masthead/logo on tiny screens

Also shrinks the border strip of red in the header on tiny screens
Merge pull request #45 from afuna/bug5073-tropo-adjustments

(Bug 5073) Tweaks to tropo (foundation version)