PLT-2257 Reorganized System Console (#3003)

* Reorganized system console

* Fixed the names of some components

* Fixed timestamp for BrandImageSetting

* Fixed merge issues

* Updated push notification settings to match master branch

* Removed top level setting pages and moved enable Gitlab/LDAP settings onto their respective pages

* Re-added restrictDirectMessage setting to system console

* Re-added email connection test and fixed some margins

* Fixed ESLint errors

* Renamed Authentication > Onboarding to Authentication > Email in the system console

* Renamed Customization > Whitelabeling to Customization > Custom Branding in System Console

* Re-added EnableOpenServer to system console

webapp/sass/layout/_headers.scss

@@ -118,7 +118,8 @@
 
 // Team Header in Sidebar
 .sidebar--left,
-.sidebar--menu {
+.sidebar--menu,
+.admin-sidebar {
     .team__header {
         @include legacy-pie-clearfix;
         padding: 9px 10px;