flex the modal header so we can remove custom styles

updates the dom order in our docs to match (floated content comes first, but no need for that in flexbox)

scss/_modal.scss

@@ -77,13 +77,11 @@
 // Modal header
 // Top section of the modal w/ title and dismiss
 .modal-header {
+  display: flex;
+  justify-content: space-between; // Put modal header elements (title and dismiss) on opposite ends
+  align-items: center; // vertically center it
   padding: $modal-header-padding;
   border-bottom: $modal-header-border-width solid $modal-header-border-color;
-  @include clearfix;
-}
-// Close icon
-.modal-header .close {
-  margin-top: -2px;
 }
 
 // Title text within header