Fixed external links to non-JIRA issue trackers

External issue links (like #123) to Redmine stopped working, this fixes what seems to be the cause.

Originally the check was `if project.used_default_issues_tracker? || !external_issues_tracker_enabled?` which was necessary because jira is external, and we need special handling for jira. However, this broke linking for other issue trackers like redmine.

This PR modifies the conditional slightly to still allow external trackers that are not jira to link