Test feature flags in management ui with selenium

Author: MarcialRosales

deps/rabbitmq_management/priv/www/js/tmpl/feature-flags.ejs

@@ -20,7 +20,7 @@
       </p>
       <button id="ff-enable-all-button" onclick='enable_all_stable_feature_flags(this);'>Enable all stable feature flags</button>
   </div>
-<div class="section">
+<div class="section" id="feature-flags">
   <h2>Feature Flags</h2>
   <div class="hider">
 <%= filter_ui(nonreq_feature_flags) %>