Fix cancel link on issuable edit form

app/views/shared/issuable/_form.html.haml

@@ -135,4 +135,4 @@
                                                                                                   method: :delete, class: 'btn btn-grouped' do
           = icon('trash-o')
           Delete
-      = link_to 'Cancel', namespace_project_issue_path(@project.namespace, @project, issuable), class: 'btn btn-grouped btn-cancel'
+      = link_to 'Cancel', polymorphic_path([@project.namespace.becomes(Namespace), @project, issuable]), class: 'btn btn-grouped btn-cancel'