[modals]: style btn-close only when in .modal-header (#1925)

* [modals]: style btn-close only when in .modal-header & .modal-content

src/scss/ui/_modals.scss

@@ -1,5 +1,5 @@
-.modal-content {
-  .btn-close {
+.modal-content, .modal-header {
+  > .btn-close {
     position: absolute;
     top: 0;
     right: 0;
@@ -64,4 +64,4 @@
 .modal-full-width {
   max-width: none;
   margin: 0 $modal-dialog-margin;
-}
+}