Add main nav (#153)

This adds a "primary nav" with 3 items:

* Get started
* Guides
* Support

"Contribute" could be added when #148 is merged.

I’ve not included "Page templates" as that page feels less valuable
longer-term (eg if the NHS website templates get moved to a plugin or
something).

Adding the primary nav possibly means that some of the breadcrumbs could
be removed or shortened (eg the "Home" breadcrumb link on Support feels
a bit odd now?), but the Service Manual website has both, so maybe it’s
fine?

There’s no "current section" indicator for now, but we can adopt that
when it gets added to the header in the next major release of NHS
frontend (hopefully).

Part of #93

app/assets/sass/main.scss

@@ -7,3 +7,10 @@
 ///////////////////////////////////////////
 // Add your custom CSS/Sass styles below...
 ///////////////////////////////////////////
+
+// This is a temporary fix to remove the 'Home' link in
+// the primary navigation which is added Header primary nav.
+// Should be able to remove this after the next NHS Frontend update.
+.nhsuk-header__navigation-item--home {
+  display: none;
+}

app/views/layout.html

@@ -13,7 +13,22 @@
     ariaLabel: "NHS Prototype Kit homepage",
     service: {
       name: serviceName
-    }
+    },
+    showNav: "true",
+    primaryLinks: [
+      {
+        label: "Get started",
+        url: "/install"
+      },
+      {
+        label: "Guides",
+        url: "/how-tos"
+      },
+      {
+        label: "Support",
+        url: "/support"
+      }
+    ]
   }) }}
 {% endblock %}