Update the OrchardCore.Users module to use both a logical and physical IDs

[kevinchalet]

The OpenID module has been updated to use a logical ID (a stringified GUID) as the "public" identifier. We should do something similar with the Users module.

Steps required to implement that:

• Add a new property in User.cs,
• Create a new Orchard-specific IUserStore interface derived from Identity's IUserStore<IUser> that exposes FindByPhysicalIdAsync() and GetPhysicalIdAsync().
• Make the Orchard UserStore implement this interface and update FindByIdAsync()/GetUserIdAsync() to use the logical ID instead of the physical ID.
• Create a new Orchard-specific UserManager class derived from Identity's UserManager<IUser> that exposes FindByPhysicalIdAsync() and GetPhysicalIdAsync().
• Update AdminController to use FindByPhysicalIdAsync where appropriate.

/cc @sebastienros

[infofromca]

I suggest we also think about

872

at this time.