Added site data initilization through APIs
Added site data initilization through APIs

refs https://github.com/TryGhost/members.js/issues/6

Use member identity and public site data admin API to fetch site data which was previously being passed down from the theme. This change allows us to be directly fetch all relevant site data except adminUrl and use it to initialize the flows.

Also adds a basic loading behavior till we finish fetching site and member logged-in data from API.

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost