Woodhouse usernames might not be GitLab users

The incident issue template contains markdown comments that woodhouse parses to build a Go template version of the file, that it uses for incident declarations via Slack. This is done to allow us to keep the template in one place, which if we want users to be able to open issues via the web, has to be here.

This change removes a hardcoded @ prefix from the username variable. If Woodhouse passes in a string that is not a GitLab username, no harm is done (but it's confusing) - but if woodhouse passes an incorrect but valid username (e.g. due to this bug https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/12417), then we will cause notifications to be sent to the wrong users.

This change allows Woodhouse to decide whether it wants to pass a GitLab handle or not - if it can - while allowing it to gracefully degrade to not @ing a random user, e.g. if the caller's Slack profile doesn't include a GitLab profile.