Image upload links not generated properly from Markdown for some users

see https://gitlab.com/gitlab-org/gitlab-ce/issues/66803

I'm adjusting to ~S2 because, as I understand it, this only applies to projects where the repository permissions are different to the issue permissions (if both were project members only there would be no issue), which limits the affected users: https://docs.gitlab.com/ee/development/contributing/issue_workflow.html#severity-labels

Marking this ~S2 and suggesting we patch gitlab.com since we have a customer ticket open https://gitlab.zendesk.com/agent/tickets/130786 (internal)