Added subscribe navigation to account home

refs https://github.com/TryGhost/members.js/issues/20

- Adds navigation and last page setup for account home -> account plan chooser

ghost/portal/src/components/pages/AccountHomePage.js

@@ -65,7 +65,10 @@ export default class AccountHomePage extends React.Component {
     }
 
     openSubscribe(e) {
-        //no-op
+        this.context.onAction('switchPage', {
+            page: 'accountPlan',
+            lastPage: 'accountHome'
+        });
     }
 
     renderAccountFooter() {