Sentry links to GL issues are wrong

The links from Sentry to a GitLab issue are wrong, but work because of https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/controllers/projects/issues_controller.rb#L200-213

This has been happening for a while: see https://sentry.gitlap.com/gitlab/gitlabcom/issues/10170/ which links to https://gitlab.com//gitlab-org/gitlab-ce/issues/2784151 when it should link to https://gitlab.com/gitlab-org/gitlab-ce/issues/21038 (no double slash after the host, and use the IID instead of the ID).