Fixes #7577: Attempt to fix, at least, issues with Windows Phone 8 and responsive not kicking in correctly

docs/assets/css/bootstrap.css

@@ -5300,6 +5300,12 @@ a.list-group-item.active > .badge,
   width: device-width;
 }
 
+@media screen and (max-width: 400px) {
+  @-ms-viewport {
+    width: 320px;
+  }
+}
+
 .hidden {
   display: none;
   visibility: hidden;

less/responsive-utilities.less

@@ -5,11 +5,27 @@
 
 // IE10 Metro responsive
 // Required for Windows 8 Metro split-screen snapping with IE10
+//
 // Source: http://timkadlec.com/2012/10/ie10-snap-mode-and-responsive-design/
 @-ms-viewport{
   width: device-width;
 }
 
+// IE10 on Windows Phone 8
+// IE10 on WP8 screws doesn't report CSS pixels, but actual device pixels.
+// In other words, say on a Lumia, you'll get 768px as the device width,
+// meaning users will see the tablet styles and not phone styles.
+//
+// Alternatively you can override this with JS (see source below), but
+// we won't be doing that here given our limited scope.
+//
+// Source: http://timkadlec.com/2013/01/windows-phone-8-and-device-width/
+@media screen and (max-width: 400px) {
+  @-ms-viewport{
+    width: 320px;
+  }
+}
+
 // Hide from screenreaders and browsers
 // Credit: HTML5 Boilerplate
 .hidden {