Add LESS variable to configure expansion of sideNav dropdowns

framework/core/less/lib/sideNav.less

@@ -9,7 +9,7 @@
 // control; the new discussion button is the primary-control. On anything
 // larger than a phone, however, we need to affix the sidebar and expand the
 // .dropdown-select into a plain list.
-@media @tablet-up {
+@media @expand-side-nav {
   .sideNav {
     // Expand the dropdown-select component into a normal nav list.
     & .Dropdown--select {
@@ -48,7 +48,10 @@
   }
 }
 
-.sideNav--horizontal {
+.sideNav--horizontal {}
+
+@media @expand-side-nav {
+  .sideNav--horizontal {
     padding: 15px 0;
     white-space: nowrap;
     overflow: auto;
@@ -83,11 +86,13 @@
     .affix {
       position: static;
     }
-}
-@media @tablet {
+  }
+
+  @media @tablet {
     .sideNav {
       .sideNav--horizontal();
     }
+  }
 }
 
 @media @desktop-up {

framework/core/less/lib/variables.less

@@ -110,6 +110,8 @@
 @zindex-alerts:                    1060;
 @zindex-tooltip:                   1070;
 
+@expand-side-nav:                  @tablet-up;
+
 // ---------------------------------
 // BREAKPOINTS