"Issue" button links to internal tracker when external tracker is used

Created by: aus

The issue button in app/views/projects/_clone_panel.html.haml links to the internal issue tracker when an external tracker has been selected. Currently, there is no support to link to the "new issue" page of an external tracker. Should this button just link to url_for_project_issues if an external project is enabled? Or perhaps just hide the button completely if an external tracker is being used? Maybe @Andrew8xx8 has some thoughts since he wrote #2967.