Check if identity was updated when updating user (Follow-up from "Exposing saml_provider_id in the users API")

The following discussion from !14045 (merged) should be addressed:

• @jprovaznik started a discussion: (+6 comments)

  Currently we don't check if identity was updated or not. WDYT about something like:

  identity = assign_identity
  identity_errors = identity&.errors || []
  
  if identity_errors.empty? && @user.save(validate: validate) && update_status
    notify_success(user_exists)
  else
    messages = identity_errors.full_messages + @user.errors.full_messages + Array(@user.status&.errors&.full_messages)
    ...

  assign_identity method would have to return identity object (instead of save/update return value):

      def assign_identity
        return if identity_params.empty?
  
        identity = find_identity(user)
        if identity
          identity.update(identity_params)
        else
          identity = user.identities.build(identity_params)
          identity.save
        end
  
        identity
      end
  

/cc @leopardm @jamedjo - could you please check/update milestone/labels?