Issue #73: Use IdentityManager's sign-in event instead of relying on application-level logic.
Issue #73: Add helper functions to PortalModel.

* Determine if a URL is from ArcGIS Online.

* Register OAuth-enabled Portal URLs.
Issue #73: Ensure HTTPS on ArcGIS Online portals.
Issue #73: Remove obsolete Portal URL cleanup method.
Issue #73: Support OAuth sign-in.
Issue #73: Register AGO OAuth infos upfront.
Merge branch '#73/support-ago-oauth' into develop