Implementation of the feature to redirect to an external issue tracker

conf/locale/locale_en-US.ini

 settings.issues_desc = Enable issue tracker
 settings.use_internal_issue_tracker = Use builtin lightweight issue tracker
 settings.use_external_issue_tracker = Use external issue tracker
+settings.external_tracker_url = External Issue Tracker URL
+settings.external_tracker_url_desc = Visitors will be redirected to URL when they click on the tab.
 settings.tracker_url_format = External Issue Tracker URL Format
 settings.tracker_issue_style = External Issue Tracker Naming Style:
 settings.tracker_issue_style.numeric = Numeric

models/repo.go

 	ExternalWikiURL       string
 	EnableIssues          bool `xorm:"NOT NULL DEFAULT true"`
 	EnableExternalTracker bool
+	ExternalTrackerURL    string
 	ExternalTrackerFormat string
 	ExternalTrackerStyle  string
 	ExternalMetas         map[string]string `xorm:"-"`

modules/auth/repo_form.go

 	ExternalWikiURL       string
 	EnableIssues          bool
 	EnableExternalTracker bool
+	ExternalTrackerURL    string
 	TrackerURLFormat      string
 	TrackerIssueStyle     string
 	EnablePulls           bool

routers/repo/issue.go

 )
 
 func MustEnableIssues(ctx *context.Context) {
-	if !ctx.Repo.Repository.EnableIssues || ctx.Repo.Repository.EnableExternalTracker {
+	if !ctx.Repo.Repository.EnableIssues {
 		ctx.Handle(404, "MustEnableIssues", nil)
 		return
 	}
+
+	if ctx.Repo.Repository.EnableExternalTracker {
+		ctx.Redirect(ctx.Repo.Repository.ExternalTrackerURL)
+		return
+	}
 }
 
 func MustAllowPulls(ctx *context.Context) {

routers/repo/setting.go

 		repo.ExternalWikiURL = form.ExternalWikiURL
 		repo.EnableIssues = form.EnableIssues
 		repo.EnableExternalTracker = form.EnableExternalTracker
+		repo.ExternalTrackerURL = form.ExternalTrackerURL
 		repo.ExternalTrackerFormat = form.TrackerURLFormat
 		repo.ExternalTrackerStyle = form.TrackerIssueStyle
 		repo.EnablePulls = form.EnablePulls

templates/repo/header.tmpl

 			<a class="{{if .PageIsViewCode}}active{{end}} item" href="{{.RepoLink}}">
 				<i class="octicon octicon-code"></i> {{.i18n.Tr "repo.code"}}
 			</a>
-			{{if and .Repository.EnableIssues (not .Repository.EnableExternalTracker)}}
+			{{if .Repository.EnableIssues}}
 				<a class="{{if .PageIsIssueList}}active{{end}} item" href="{{.RepoLink}}/issues">
-					<i class="octicon octicon-issue-opened"></i> {{.i18n.Tr "repo.issues"}} <span class="ui {{if not .Repository.NumOpenIssues}}gray{{else}}blue{{end}} small label">{{.Repository.NumOpenIssues}}</span>
+					<i class="octicon octicon-issue-opened"></i> {{.i18n.Tr "repo.issues"}} {{if not .Repository.EnableExternalTracker}}<span class="ui {{if not .Repository.NumOpenIssues}}gray{{else}}blue{{end}} small label">{{.Repository.NumOpenIssues}}{{end}}</span>
 				</a>
 			{{end}}
 			{{if .Repository.AllowsPulls}}

templates/repo/settings/options.tmpl

 								</div>
 							</div>
 							<div class="field {{if not .Repository.EnableExternalTracker}}disabled{{end}}" id="external_issue_box">
+								<div class="field">
+									<label for="external_tracker_url">{{.i18n.Tr "repo.settings.external_tracker_url"}}</label>
+									<input id="external_tracker_url" name="external_tracker_url" type="url" value="{{.Repository.ExternalTrackerURL}}">
+									<p class="help">{{.i18n.Tr "repo.settings.external_tracker_url_desc"}}</p>
+								</div>
 								<div class="field">
 									<label for="tracker_url_format">{{.i18n.Tr "repo.settings.tracker_url_format"}}</label>
 									<input id="tracker_url_format" name="tracker_url_format" type="url" value="{{.Repository.ExternalTrackerFormat}}" placeholder="e.g. https://github.com/{user}/{repo}/issues/{index}">