Fixes #12470: Scope large modal styles to minimum viewport width

dist/css/bootstrap.css

@@ -5197,6 +5197,8 @@ button.close {
   .modal-sm {
     width: 300px;
   }
+}
+@media (min-width: 992px) {
   .modal-lg {
     width: 900px;
   }

less/modals.less

@@ -121,7 +121,6 @@
 
 // Scale up the modal
 @media (min-width: @screen-sm-min) {
-
   // Automatically set modal's width for larger viewports
   .modal-dialog {
     width: @modal-md;
@@ -133,6 +132,8 @@
 
   // Modal sizes
   .modal-sm { width: @modal-sm; }
-  .modal-lg { width: @modal-lg; }
+}
 
+@media (min-width: @screen-md-min) {
+  .modal-lg { width: @modal-lg; }
 }