Don't require app-name for Firebase elements

[arjunyel]

Regular Polymerfire elements find the name of the firebase-app automatically (as long as their is just one firebase-app) so the convoo fire elements should replicate this behavior. If you use, for example file-fire, you are required to set app-name on every other polymerfire element (such as firebase-auth) and a name on firebase-app.

[sbeleidy]

Agreed - thanks for creating the issue 😄

We just have to figure out how to set the default value of app-name so that it works with the default app without specifying anything while allowing those who need it to be able to specify the app name.

Right now, we require the app-name so that we could handle the case when someone wants to specify the name. But this means that we end up forcing users to use an app-name on everything they work with which is not a great experience.