Allow use to set an external tracker url and to redirect user when clicking in Issues same behavior as Wiki. (#3646)

modules/auth/repo_form.go

@@ -96,6 +96,7 @@ type RepoSettingForm struct {
 	ExternalWikiURL       string
 	EnableIssues          bool
 	EnableExternalTracker bool
+	ExternalTrackerURL    string
 	TrackerURLFormat      string
 	TrackerIssueStyle     string
 	EnablePulls           bool