Move menu button to the left in the admin section

So now it uses the same interface and styles as the main layout. On
small screens, it's easier to play with the menu when the button is on
the left because the menu it opens is aligned to the left.

Note that now we can get rid of the title-bar class; we didn't use the
styles in the public area since commit dcec003d0, and we were overriding
all the Foundation styles in the admin area with the exception of the
padding, which we no longer need.

app/assets/stylesheets/layout/admin_header.scss

@@ -77,10 +77,4 @@
       height: auto !important;
     }
   }
-
-  .title-bar {
-    color: inherit;
-    position: absolute;
-    right: 12px;
-  }
 }