@Tomi The adminTemplate has been removed, we highly recommend creating a custom page using "Edit pages" or "Dashboard designer" (you will find these in the menu). You can set them up so they are full screen pages, i.e. have no left menu or toolbar at the top.
If you do want to create your own custom AngularJS project and serve it from the Mango web server you can still do so. However the onus is on you to maintain it. The old adminTemplate is located here as a starting point - https://github.com/infiniteautomation/ma-dashboards/tree/3.3.x/UI/web/adminTemplate