Fixed upgrade modal cta when activating a theme

refs https://github.com/TryGhost/Team/issues/590

- Previous upgrade button behavior was opening up a new browser window leading to now retired billing site. Tha behavior change looks like an oversight we missed to cleanup when introducing an in-admin billing pabe "/pro"
- The change follows similar pattern used in other limit check mesages where tha "upgrade" button takes the user to the billing page

ghost/admin/app/components/modal-upgrade-host-limit-custom-theme.js

@@ -1,12 +1,12 @@
 import ModalComponent from 'ghost-admin/components/modal-base';
+import {inject as service} from '@ember/service';
 
 export default ModalComponent.extend({
+    router: service(),
+
     actions: {
         upgrade: function () {
-            const upgradeLink = this.model.upgradeLink;
-            window.open(upgradeLink);
-            this.closeModal();
-            return true;
+            this.router.transitionTo('pro');
         }
     }
 });