FIX: uses simpler pattern for custom href on extra nav items (#8015)

THe main advantage of this solution is that it will be called on each rerendered whereas the other is not once href has been set.

Example API:
```
      api.addNavigationBarItem({
        name: "foo",
        displayName: "Foo",
        customHref: function(category, args) {
          const router = api.container.lookup("service:router");
          const queryParams = { bar: "1" };
          return router.urlFor(router.currentRouteName, category, {
            queryParams
          });
        }
      });
```

app/assets/javascripts/discourse/models/nav-item.js.es6

@@ -101,18 +101,8 @@ const NavItem = Discourse.Model.extend({
 });
 
 const ExtraNavItem = NavItem.extend({
-  href: Ember.computed({
-    set(key, value) {
-      let customHref;
-      NavItem.customNavItemHrefs.forEach(function(cb) {
-        customHref = cb.call(this, this);
-        if (customHref) {
-          return false;
-        }
-      }, this);
-      return customHref || value;
-    }
-  }),
+  @computed("href")
+  href: href => href,
 
   customFilter: null
 });
@@ -189,6 +179,11 @@ NavItem.reopenClass({
       return item.customFilter.call(this, category, args);
     });
 
+    extraItems.forEach(item => {
+      if (!item.customHref) return;
+      item.set("href", item.customHref.call(this, category, args));
+    });
+
     return items.concat(extraItems);
   }
 });