Add .modal--center modifier

This can potentially be used for other purposes as well, but for now, I have used it to center the `Player` inside the `PlayerModal` in dpl-react.

src/stories/Library/Modals/modal.scss

@@ -70,6 +70,11 @@
   padding: 0;
 }
 
+.modal--center {
+  justify-content: center;
+  align-items: center;
+}
+
 .modal-btn-close {
   background-color: $color__global-primary;
   position: fixed;