Search / filter issues does not search issue comments

This can make it difficult to find existing issues. For example if I search "oauth scope" or "oauth scopes" I find nothing for GitLab CE Issues, but there is something which clearly should match: https://gitlab.com/gitlab-org/gitlab-ce/issues/20492

Related: People create issues when they can't find their issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/20851