Fixed missing CSS on published site = adding bundling.

Justification:
When published (under Release configuration) in layout there is just app.min.css. We can either add also other minified CSS there (components, headers,...) or add one bundled and minified 8KB css.

src/Web/Views/Shared/_Layout.cshtml

@@ -20,7 +20,7 @@
         <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.0.0-alpha.5/css/bootstrap.min.css"
               asp-fallback-href="~/lib/bootstrap/dist/css/bootstrap.min.css"
               asp-fallback-test-class="sr-only" asp-fallback-test-property="position" asp-fallback-test-value="absolute" />
-        <link rel="stylesheet" href="~/css/app.min.css" asp-append-version="true" />
+        <link rel="stylesheet" href="~/css/site.min.css" asp-append-version="true" />
     </environment>
 </head>
 <body>