Web Diagramming tool

My team and I were assigned a project, in which we are required to build web diagramming tool that will utilize the concept of user accounts and storage/saving of diagrams. Almost all of the people in my team have never had any experience programming web-apps or any web-dev projects before. For starter we have looked at examples like draw.io and lucidcharts.com. we all know that we need use HTML, CSS and JavaScript. Moreover most of us have good grasp of how things are done in PostgreSQL. However, we are not sure about the server side because none of us are familiar with any back-end programming. The issue that is causing a lot of stress to us is that fact that we are not sure on which languages to use to implement the tool in.

TL;DR what languages/frameworks/APIs does the community recommend in order to build such a tool.