Fix #1078 (#1111)

* Rearrange responsive typography definition. Fixes #1078

* Renamed mixin (compliant with the styleguide)

sass/base/helpers.sass

@@ -21,28 +21,31 @@
 
 // Typography
 
+=typography-size($target:'')
   @each $size in $sizes
     $i: index($sizes, $size)
-  .is-size-#{$i}
+    .is-size-#{$i}#{if($target == '', '', '-' + $target)}
       font-size: $size !important
+
++typography-size()
+
 +mobile
-    .is-size-#{$i}-mobile
+  +typography-size('mobile')
-      font-size: $size !important
+
 +tablet
-    .is-size-#{$i}-tablet
+  +typography-size('tablet')
-      font-size: $size !important
+
 +touch
-    .is-size-#{$i}-touch
+  +typography-size('touch')
-      font-size: $size !important
+
 +desktop
-    .is-size-#{$i}-desktop
+  +typography-size('desktop')
-      font-size: $size !important
+
 +widescreen
-    .is-size-#{$i}-widescreen
+  +typography-size('widescreen')
-      font-size: $size !important
+
 +fullhd
-    .is-size-#{$i}-fullhd
+  +typography-size('fullhd')
-      font-size: $size !important
 
 $alignments: ('centered': 'center', 'left': 'left', 'right': 'right')