Merge pull request #12249 from twbs/modal-md

add & use @modal-md Less var for uniformity

less/modals.less

@@ -124,7 +124,7 @@
 
   // Automatically set modal's width for larger viewports
   .modal-dialog {
-    width: 600px;
+    width: @modal-md;
     margin: 30px auto;
   }
   .modal-content {

less/variables.less

@@ -561,6 +561,7 @@
 @modal-footer-border-color:   @modal-header-border-color;
 
 @modal-lg:                    900px;
+@modal-md:                    600px;
 @modal-sm:                    300px;