Redirect to external issue tracker from `/issues`
Prior, in order to display the correct link to "Issues" in the project navigation, we were performing a check against the project to see if it used an external issue tracker, and if so, we used that URL. This was inefficient.
Now, we simply always link to namespace_project_issues_path
, and
then in the controller we redirect to the external tracker if it's
present.
This also removes the need for the url_for_issue
helper. Bonus!