Add .is-right to .navbar-dropdown (#941)

As it is possible to have `is-right` on a `dropdown`, it should be possible to have this class on `navbar-dropdown` as well.

sass/components/navbar.sass

@@ -217,6 +217,9 @@ a.navbar-item,
       transform: translateY(-5px)
       transition-duration: $speed
       transition-property: opacity, transform
+    &.is-right
+      left: auto
+      right: 0
   .navbar-divider
     display: block
   .container > .navbar