revert(tabs): undo commit that removes tab sub pages

Fix advanced tabs test

This reverts
https://github.com/driftyco/ionic/commit/743de19ae898e83375b51ef9c376225
c8e63f0ef

src/components/app/structure.scss

@@ -219,6 +219,7 @@ ion-content.js-scroll > scroll-content {
 
 [nav-viewport],
 [nav-portal],
+[tab-portal],
 .nav-decor {
   display: none;
 }