[Fixes #5728] Adding default grid system to TheThemeMachine

Breaking change: Existing themes relying on Layout and TheThemeMachine
will need to add a reference to this css.

src/Orchard.Web/Modules/Orchard.Layouts/Views/EditorTemplates/LayoutEditor.cshtml

@@ -26,6 +26,9 @@
     Style.Include("LayoutEditor.css", "LayoutEditor.min.css");
     Script.Include("LayoutDesignerHost.js");
 
+    // The grid system
+    Style.Include("default-grid.css");
+
     using (Script.Foot()) {
         <script>
             angular

src/Orchard.Web/Modules/Orchard.Layouts/Views/Parts.Layout.cshtml

@@ -1,6 +1,3 @@
-@{
-    Style.Include("default-grid.css");
-}
-<div class="orchard-layouts-root">
+<div class="orchard-layouts-root">
     @Display(Model.LayoutRoot)
 </div>