Implement `archive_on_destroy` attribute for projects. Closes #761

This change implements an archive_on_destroy attribute on the gitlab_project resource. It defaults to false, if set to true, on a destroy it will archive the project instead of deleting it.

This is especially useful in organizations where repositories need to be kept for regulatory purposes and can't be deleted.

PS: the diff is a mess: I've basically just if'ed the DeleteProject case depending on the archive_on_destroy attribute and called ArchiveProject in the new branch :)