enable button groups in modal-footer by removing 5px added margin between .btn

docs/assets/css/bootstrap.css

@@ -3323,6 +3323,9 @@ input[type="submit"].btn.btn-small {
   margin-left: 5px;
   margin-bottom: 0;
 }
+.modal-footer .btn-group .btn + .btn {
+  margin-left: -1px;
+}
 .tooltip {
   position: absolute;
   z-index: 1020;

less/modals.less

@@ -83,4 +83,8 @@
     margin-left: 5px;
     margin-bottom: 0; // account for input[type="submit"] which gets the bottom margin like all other inputs
   }
+  // but override that for button groups
+  .btn-group .btn + .btn {
+    margin-left: -1px;
+  }
 }