feat(typography): updates font to Open Sans

The designs were in Open Sans and the site was in Source Sans Pro and... it got ugly.

[#115903383]

BREAKING CHANGE: the fonts are all different now; Open Sans is bigger than Source Sans Pro at the same font size...

src/pivotal-ui/components/pui-variables.scss

@@ -317,7 +317,7 @@ $link-inverse-color:                      $accent-5;
 // Typography
 // -------------------------
 
-$font-family-sans-serif:  'Source Sans Pro', 'SourceSansPro', sans-serif !default;
+$font-family-sans-serif:  'Open Sans', 'OpenSans', sans-serif !default;
 $font-family-serif:       Georgia, "Times New Roman", Times, serif !default;
 $font-family-monospace:   Menlo, Consolas, "Courier New", monospace !default;
 $font-family-base:        $font-family-sans-serif !default;
@@ -330,10 +330,10 @@ $font-size-small:         13px;
 $font-size-extra-small:   12px;
 
 
-$font-weight-em-low:      200;
+$font-weight-em-low:      300;
 $font-weight-em-default:  400;
-$font-weight-em-high:     600;
-$font-weight-em-max:      900;
+$font-weight-em-high:     700;
+$font-weight-em-max:      800;
 
 
 $headings-font-family:    null;

src/pivotal-ui/components/typography/typography.scss

@@ -2,7 +2,7 @@
 
 @mixin font-helper($name, $weight, $style) {
   @font-face {
-    font-family: 'SourceSansPro';
+    font-family: 'OpenSans';
     src: url("fonts/#{$name}.eot");
     src: url("fonts/#{$name}.eot?#iefix") format("embedded-opentype"),
     url("fonts/#{$name}.woff") format("woff"),
@@ -12,11 +12,12 @@
     font-style: $style;
   }
 }
-@include font-helper('sourcesanspro-light-webfont', 200, normal);
-@include font-helper('sourcesanspro-regular-webfont', 400, normal);
-@include font-helper('sourcesanspro-it-webfont', 400, italic);
-@include font-helper('sourcesanspro-bold-webfont', 600, normal);
-@include font-helper('sourcesanspro-black-webfont', 900, normal);
+@include font-helper('OpenSans-Light-webfont', 300, normal);
+@include font-helper('OpenSans-Regular-webfont', 400, normal);
+@include font-helper('OpenSans-Italic-webfont', 400, italic);
+@include font-helper('OpenSans-Semibold-webfont', 600, normal);
+@include font-helper('OpenSans-Bold-webfont', 700, normal);
+@include font-helper('OpenSans-ExtraBold-webfont', 800, normal);
 
 /*doc
 ---