Added support for Tiers API to the get helper

https://github.com/TryGhost/Team/issues/1405

This will allow themes to fetch the list of tiers for creating custom
signup pages.

core/frontend/helpers/get.js

@@ -31,6 +31,9 @@ const RESOURCES = {
     },
     authors: {
         alias: 'authorsPublic'
+    },
+    tiers: {
+        alias: 'tiersPublic'
     }
 };