updated container mixin to be fixed-container instead so we can keep the .container in scaffolding while removing the repeated code

lib/preboot.less

@@ -121,9 +121,10 @@
 }
 
 // Grid System
-.container {
+.fixed-container {
   width: @siteWidth;
-  margin: 0 auto;
+  margin-left: auto;
+  margin-left: auto;
   .clearfix();
 }
 .columns(@columnSpan: 1) {

lib/scaffolding.less

@@ -73,9 +73,7 @@ body {
 
 // Container (centered, fixed-width layouts)
 .container {
-  width: @siteWidth;
-  margin-left: auto;
-  margin-right: auto;
+  .fixed-container();
 }
 
 // Fluid layouts (left aligned, with sidebar, min- & max-width content)