Add percentage rollout support to feature flag UI (CE)

This commit updates the feature flags pages to includes support for the
percentage rollout strategy.

app/assets/stylesheets/framework/common.scss

@@ -435,6 +435,7 @@ img.emoji {
 /** COMMON SIZING CLASSES **/
 .w-0 { width: 0; }
 .w-8em { width: 8em; }
+.w-3rem { width: 3rem; }
 
 .h-12em { height: 12em; }