[MM-46694] A/B Test: welcome post (#20926)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/sqlstore/user_store.go

@@ -1748,6 +1748,16 @@ func (us SqlUserStore) InferSystemInstallDate() (int64, error) {
 	return createAt, nil
 }
 
+func (us SqlUserStore) GetFirstSystemAdminID() (string, error) {
+	var id string
+	err := us.GetReplicaX().Get(&id, "SELECT Id FROM Users WHERE Roles LIKE ? ORDER BY CreateAt ASC LIMIT 1", "%system_admin%")
+	if err != nil {
+		return "", errors.Wrap(err, "failed to get first system admin")
+	}
+
+	return id, nil
+}
+
 func (us SqlUserStore) GetUsersBatchForIndexing(startTime int64, startFileID string, limit int) ([]*model.UserForIndexing, error) {
 	users := []*model.User{}
 	usersQuery, args, err := us.usersQuery.