Define modal first and last child border-radius

Use $modal-border-radius for all modal border radius definitions

app/assets/stylesheets/framework/modal.scss

@@ -87,7 +87,17 @@ body.modal-open {
   background-color: $black-transparent;
   .modal-content {
-    border-radius: 0.25rem;
+    border-radius: $modal-border-radius;
+
+    *:first-child {
+      border-top-left-radius: $modal-border-radius;
+      border-top-right-radius: $modal-border-radius;
+    }
+
+    *:last-child {
+      border-bottom-left-radius: $modal-border-radius;
+      border-bottom-right-radius: $modal-border-radius;
+    }
   }
   @include media-breakpoint-up(sm) {

app/assets/stylesheets/framework/variables.scss

@@ -807,6 +807,7 @@ Modals
 */
 $modal-body-height: 80px;
 $modal-border-color: #e9ecef;
+$modal-border-radius: 0.25rem;
 $priority-label-empty-state-width: 114px;