Add "Subscribe to Feed" page

First attempt on #5. There are still a lot of things to do, but I was hoping we can use this for discussion and questions.

So far I have created a separate page to add a subscription and linked it from the categories page. This page contains a form that asks for the URL of the feed, and once submitted it will instantiate the feed model and trigger the `subscribeToFeed` API method using the `create` sync method.

Right now I'm letting the form refresh the app to get back to the main page. How would you route to another page from the view layer?