fix(activity): move initialization to the constructor

$onInit method is not called in a service: activities array was
undefined and in streaming loop the refresh call raised an exception
hidden in the empty catch.

Bug introduced in the massive changes for webpack.

src/client/app/components/activity/activity.service.js

@@ -3,9 +3,7 @@ export class ActivityService {
         this.$http = $http;
         this.SessionService = SessionService;
         this.AccountsService = AccountsService;
-    }
 
-    $onInit() {
         this.activities = [];
     }