use include directive for badge demo pages

src/badge/snippets/badge-on-icon-icon-demo.html

@@ -3,5 +3,4 @@
   color: rgba(0, 0, 0, 0.24);
 }
 </style>
-<!-- Icon badge on icon -->
-<div class="icon material-icons mdl-badge badge-demo" data-badge="♥">account_box</div>
+{% include "badge-on-icon-icon.html" %}

src/badge/snippets/badge-on-icon-text-demo.html

@@ -2,5 +2,5 @@
 .badge-demo {
   color: rgba(0, 0, 0, 0.24);
 }
-</style><!-- Number badge on icon -->
-<div class="icon material-icons mdl-badge badge-demo" data-badge="1">account_box</div>
+</style>
+{% include "badge-on-icon-text.html" %}

src/badge/snippets/badge-on-text-icon-demo.html

@@ -3,5 +3,4 @@
   color: rgba(0, 0, 0, 0.24);
 }
 </style>
-<!-- Icon badge -->
-<div class="mdl-badge badge-demo" data-badge="♥">Mood</div>
+{% include "badge-on-text-icon.html" %}

src/badge/snippets/badge-on-text-text-demo.html

@@ -3,5 +3,4 @@
   color: rgba(0, 0, 0, 0.24);
 }
 </style>
-<!-- Number badge -->
-<div class="mdl-badge badge-demo" data-badge="4">Inbox</div>
+{% include "badge-on-text-text.html" %}