navmodel: fix for signout link on pref page

without a target property on the link, the signout tab on
the preferences page returned a 404.

public/app/core/components/PageHeader/PageHeader.tsx

@@ -19,7 +19,7 @@ function TabItem(tab: NavModelItem) {
 
   return (
     <li className="gf-tabs-item" key={tab.url}>
-      <a className={tabClasses} href={tab.url}>
+      <a className={tabClasses} target={tab.target} href={tab.url}>
         <i className={tab.icon} />
         {tab.text}
       </a>

public/app/core/nav_model_srv.ts

@@ -12,6 +12,7 @@ export interface NavModelItem {
   hideFromTabs?: boolean;
   divider?: boolean;
   children: NavModelItem[];
+  target?: string;
 }
 
 export class NavModel {