Update Bootstrap and jQuery to the latest compatible versions

jQuery 3.x works with Bootstrap 3.3.7 and above.
This also switches to the jQuery Slim version as we don't use the
Ajax or Effects components from jQuery.

bower.json

@@ -13,7 +13,7 @@
     "tests"
   ],
   "dependencies": {
-    "bootstrap": "3",
-    "blueimp-file-upload": "jquery-file-upload#^9.12.3"
+    "bootstrap": "3.4.1",
+    "jquery": "3.5.1"
   }
 }

templates/_footer.phtml

@@ -33,9 +33,9 @@
     </div>
 
     <!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
-    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>
+    <script defer src="<?php echo raw($bowerpath) ?>/jquery/dist/jquery.slim.min.js"></script>
     <!-- Include all compiled plugins (below), or include individual files as needed -->
-    <script src="<?php echo raw($bowerpath) ?>/bootstrap/dist/js/bootstrap.min.js"></script>
+    <script defer src="<?php echo raw($bowerpath) ?>/bootstrap/dist/js/bootstrap.min.js"></script>
 </body>
 
 </html>