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 puts the jira check first, moving the original check (and removing the `!` from the `external_issues_tracker_enabled?` check) into the else if.
Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.