Add gitea/issues link to 500 page (#4654)

templates/status/500.tmpl

 	<br>
 	{{if .ErrorMsg}}<p>An error has occurred : {{.ErrorMsg}}</p>{{end}}
 	{{if .ShowFooterVersion}}<p>Application Version: {{AppVer}}</p>{{end}}
+	{{if .IsAdmin}}<p>If you are sure this is Gitea bug, please search for issue on <a href="https://github.com/go-gitea/gitea/issues">GitHub</a> and open new issue if necessary.</p>{{end}}
 </div>
 {{template "base/footer" .}}