Fix mentioning of private groups.

issue.participants would mistakenly not send the current_user to mentioned_users, which means it would search for referenced groups and users as if the user was signed out. Since signed out users don't have access to private groups (groups without public projects) and since you can't reference a group you don't have access to (since this would leak its existence), the group reference wasn't found and the group members weren't notified.

Addresses internal issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2326.