Chore: Move user errors to user service (#52460)

* Move user not found err to user service

* User ErrCaseInsensitive from user pkg

* User ErrUserAlreadyExists from user pkg

* User ErrLastGrafanaAdmin from user pkg

* Remove errors from model

pkg/services/serviceaccounts/database/database.go

@@ -72,7 +72,7 @@ func (s *ServiceAccountsStoreImpl) CreateServiceAccount(ctx context.Context, org
 	})
 
 	if createErr != nil {
-		if errors.Is(createErr, models.ErrUserAlreadyExists) {
+		if errors.Is(createErr, user.ErrUserAlreadyExists) {
 			return nil, ErrServiceAccountAlreadyExists
 		}