Fix settings submit button position on wide screens

Closes #89 (closed)

src/views/settings/components/Instance.vue

 <template>
-  <div v-if="!loading" class="form-container">
+  <div v-if="!loading" :class="isSidebarOpen" class="form-container">
     <el-form ref="instanceData" :model="instanceData" :label-width="labelWidth">
       <setting :setting-group="instance" :data="instanceData"/>
     </el-form>
@@ -82,6 +82,9 @@ export default {
     isMobile() {
       return this.$store.state.app.device === 'mobile'
     },
+    isSidebarOpen() {
+      return this.$store.state.app.sidebar.opened ? 'sidebar-opened' : 'sidebar-closed'
+    },
     isTablet() {
       return this.$store.state.app.device === 'tablet'
     },