AddReaderAccess fix, added tests, refactoring, fix of cmdlet

AddReaderAccess is now language independent. The correct claims identity is now used to add "Everyone but external users".
Added several SecurityExtensions tests
Moved Web.AddAdministratorsTenant to Tenant.AddAdministrators and marked the Web.AddAdministratorsTenant method as obsolete.
Fixed Remove-SPOField cmdlet to also allow removal of site columns.