Remove experimental code.
Some default songs.
Login component.
Remove comment.
Refactor localStorage logic into own class.

And simplify app data flow. App model is stored in stores. Stores are
persisted into localStorage when available. Storage handles app data
flow via events from stores.
app.Firebase stub.
Many changes.

- Use locally generated id for song in url because name and artist
should be editable for not yet published songs.
- Fix sync whole app model across browser tabs/windows.
- Simplify app data flow. Stores hold app state.
- Refactoring and bug fixing.