Extend azuredevops_user_entitlement to support import and update (#227)

* azuredevops_user_entitlement now supports import via UUID and UPN and supports to update an entitlement

* Added method getAPIErrorMessage to azuredevops\resource_user_entitlement.go

* Added new unit tests to azuredevops\resource_user_entitlement_test.go

* Changed documentation website\docs\r\user_entitlement.html.markdown

* Fixed again some formatting issues  in azuredevops/resource_user_entitlement_test.go and azuredevops/resource_user_entitlement.go

* Added missing import statements to azuredevops\utils\converter\converter.go

* Extended DiffSuppressFunc in azuredevops/resource_user_entitlement.go so that the entitlements 'earlyAdopter', 'express' and 'basic' are treated as equal

Co-authored-by: Thomas Meckel <tmeckel@users.noreply.github.com>

Author: tmeckel