addressing #9189 manually, part 6: add a navbar margin var, but avoid the...

addressing #9189 manually, part 6: add a navbar margin var, but avoid the navbar-nav var because that should really be customizable

less/navbar.less

@@ -6,7 +6,7 @@
 .navbar {
   position: relative;
   min-height: @navbar-height; // Ensure a navbar always shows (e.g., without a .navbar-brand in collapsed mode)
-  margin-bottom: 20px;
+  margin-bottom: @navbar-margin-bottom;
   padding-left: @navbar-padding-horizontal;
   padding-right: @navbar-padding-horizontal;
   background-color: @navbar-bg;

less/variables.less

@@ -232,6 +232,7 @@
 @navbar-height:                    50px;
 @navbar-color:                     #777;
 @navbar-bg:                        #eee;
+@navbar-margin-bottom:             @line-height-computed;
 @navbar-border-radius:             @border-radius-base;
 @navbar-padding-horizontal:        floor(@grid-gutter-width / 2);
 @navbar-padding-vertical:          ((@navbar-height - @line-height-computed) / 2);