Cleanup work

Remove User POCO and switch to using shared IdentityUser for EF
Rename claimstype options to ClaimsIdentityOptions
Cleanup overloads for IdentityBuilder
Passing options objects like this seems a bit strange. Makes me wonder if someone else is responsible from resolving the IOptionsAccessor<IdentityOptions> so that the "ClaimsIdentityOptions" instance can be passed here or if the constructor of the concrete IClaimsIdentityFactory should get through injection.