Add is-halfheight to hero (#783)

* Add is-halfheight to hero

* Remove duplication

sass/layout/hero.sass

@@ -142,11 +142,15 @@
       .hero-body
         padding-bottom: 18rem
         padding-top: 18rem
+  &.is-halfheight,
   &.is-fullheight
-    min-height: 100vh
     .hero-body
       align-items: center
       display: flex
       & > .container
         flex-grow: 1
         flex-shrink: 1
+  &.is-halfheight
+    min-height: 50vh
+  &.is-fullheight
+    min-height: 100vh