Set Singly ID and Secret in application template

The Singly ID and Secret are needed in [`users_controller.rb#L19`](https://github.com/1kplus/reader/blob/e9c5acd465111567fe14db686a5eddb621db774e/app/controllers/users_controller.rb#L19) and specs fail without it.

This PR adds the Singly stuff to `application.yml.template` so it's added when we run `script/setup`