Add progressive enhancement to service switcher

- by default, the menu is open
- if Javascript is enabled/loaded, the links are hidden, and visual cues (▶) to
  show that it can be opened are added
- clicking it opens and closes it

app/assets/stylesheets/components/dropdown.scss

@@ -0,0 +1,53 @@
+%dropdown,
+.dropdown {
+
+  position: relative;
+
+  .js-enabled &-toggle {
+
+    color: $link-colour;
+    position: relative;
+    padding-left: 20px;
+    cursor: pointer;
+
+    &:hover {
+      color: $link-hover-colour;
+      text-decoration: underline;
+    }
+
+    &::before {
+      content: '▼';
+      font-size: 12px;
+      position: absolute;
+      top: 1px;
+      left: -1px;
+    }
+
+  }
+
+  .js-closed &-toggle::before {
+    content: '▶';
+    left: -1px;
+    top: 2px;
+  }
+
+  a {
+
+    display: block;
+
+    .js-enabled & {
+      padding-left: 20px;
+    }
+
+  }
+
+  &.js-closed {
+
+    a {
+      left: -9999em;
+      position: absolute;
+    }
+
+  }
+
+}

app/assets/stylesheets/components/management-navigation.scss

@@ -1,7 +1,7 @@
 .management-navigation {
 
   @extend %site-width-container;
-  @include core-19();
+  @include core-16();
   border-bottom: 1px solid $border-colour;
   padding: 10px 0 8px;