rabbit_feature_flags: Hide required feature flags in management UI

[Why]
They just add noise to the UI and there is nothing the user can do about
them at that point.

Given their number will only increase, let's hide them to let the user
focus on the feature flags they can act on.

Author: dumbbell

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

@@ -30,9 +30,14 @@
     <%
        for (var i = 0; i < feature_flags.length; i++) {
          var feature_flag = feature_flags[i];
-	 if (feature_flag.stability == "experimental") {
-	   continue;
-	 }
+         if (feature_flag.stability == "required") {
+           /* Hide required feature flags. There is nothing the user can do
+            * about them and they just add noise to the UI. */
+           continue;
+         }
+         if (feature_flag.stability == "experimental") {
+           continue;
+         }
          var state_color = "grey";
          if (feature_flag.state == "enabled") {
            state_color = "green";
@@ -103,9 +108,9 @@ These flags can be enabled in production deployments after an appropriate amount
     <%
        for (var i = 0; i < feature_flags.length; i++) {
          var feature_flag = feature_flags[i];
-	 if (feature_flag.stability != "experimental") {
-	   continue;
-	 }
+         if (feature_flag.stability != "experimental") {
+           continue;
+         }
          var state_color = "grey";
          if (feature_flag.state == "enabled") {
            state_color = "green";
@@ -119,14 +124,14 @@ These flags can be enabled in production deployments after an appropriate amount
          <td><%= fmt_string(feature_flag.name) %></td>
          <td class="c">
            <% if (feature_flag.state == "disabled") { %>
-	      <div>
+              <div>
               <input id="<%= feature_flag.name %>" type="checkbox" class="riskCheckbox" onclick="this.parentNode.querySelector('.enable-feature-flag input[type=submit]').disabled = !this.checked;">
               <label for="<%= feature_flag.name %>">I understand that once enabled, this feature flag cannot be disabled</label><br>
-	      <br>
+              <br>
               <form action="#/feature-flags-enable" method="put" style="display: inline-block" class="enable-feature-flag">
               <input type="hidden" name="name" value="<%= fmt_string(feature_flag.name) %>"/>
               <input type="submit" value="Enable" class="c" disabled="disabled"/>
-	      </div>
+              </div>
            </form>
            <% } else { %>
            <abbr class="status-<%= fmt_string(state_color) %>"