Fix system menu of plugins (#3902)

In the system settings, the menu-item-hover class has been added to the plugin menu.

src/main/twirl/gitbucket/core/admin/menu.scala.html

@@ -33,7 +33,7 @@
       </li>
       @gitbucket.core.plugin.PluginRegistry().getSystemSettingMenus.map { menu =>
         @menu(context).map { link =>
-          <li@if(active==link.id){ class="active"}>
+          <li class="menu-item-hover @if(active==link.id){active}">
             <a href="@context.path/@link.path">
               <i class="menu-icon octicon octicon-@link.icon.getOrElse("plug")"></i>
               <span>@link.label</span>