fix(network): Fix network manipulation together with Bootstrap styles (#2654)

fixes #2640

lib/network/css/network-manipulation.css

@@ -1,4 +1,6 @@
 div.vis-network div.vis-manipulation {
+  box-sizing: content-box;
+
   border-width: 0;
   border-bottom: 1px;
   border-style:solid;
@@ -146,4 +148,4 @@ div.network-navigation_wrapper {
   width: 100%;
   height: 100%;
 }
-*/
+*/