Add a new modifier class for disabling resizability of a textarea (#1045)

sass/elements/form.sass

@@ -98,6 +98,9 @@ $help-size: $size-small !default
     min-height: 120px
   &[rows]
     height: unset
+  // Modifiers
+  &.has-fixed-size
+    resize: none
 
 .checkbox,
 .radio