Add variable for content max width

app/assets/stylesheets/framework/variables.scss

@@ -169,6 +169,7 @@ $header-height: 50px;
 $fixed-layout-width: 1280px;
 $limited-layout-width: 990px;
 $limited-layout-width-sm: 790px;
+$container-text-max-width: 540px;
 $gl-avatar-size: 40px;
 $error-exclamation-point: $red-500;
 $border-radius-default: 3px;