feat:新增功能

1.Dashboard下增加子菜单：Panel、MetricPreview
2.Panel菜单下完成以下功能
   （1）panel列表展示中的新增功能
   （2）panel列表展示中对应记录的删除和修改功能
   （3）Add按钮的新增panel功能
3.MetricPreview菜单完成了左侧列表的跳转功能

nezha-fronted/src/components/common/header.vue

@@ -23,11 +23,17 @@
                         </el-menu-item>
                     </template>
                 </el-submenu>
-                <el-menu-item index="1" @click="jumpTo('dashboard')">
-                    <div>
-                        {{$t('overall.dashboard')}}
-                    </div>
-                </el-menu-item>
+                <el-submenu index="1"  popper-class="nz-submenu">
+                    <template slot="title">
+                      <div @click="jumpTo('dashboard')">{{$t('overall.dashboard')}}</div>
+                    </template>
+                    <el-menu-item index="1-0">
+                      <div @click="jumpTo('panel')">{{$t('dashboard.panel.title')}}</div>
+                    </el-menu-item>
+                    <el-menu-item index="1-1">
+                      <div @click="jumpTo('metricPreview')">{{$t('dashboard.metricPreview.title')}}</div>
+                    </el-menu-item>
+                </el-submenu>
                 <el-submenu index="2" popper-class="nz-submenu">
                     <template slot="title">
                         <div @click="jumpToProject(projectData[0])">{{$t('overall.project')}}</div>