Move out code of backend in new repository

[c8y3]

The goal of this issue is to split the iris-web repository in order to move out all code related to the backend.
That way, we will have the following setup:

• iris-backend: the API server. It contains the database and app dockerfiles, a development and CI docker-compose. Releases the db and app docker images
• iris-frontend: the new application frontend. It contains a development and CI docker-compose. Releases the frontend docker images
• iris-web: assembly of the whole project, released to the end-user. With nginx dockerfile, production docker-compose and e2e tests.
  That way backend and frontend can be developed in parallel, released (on tag) and integrated in iris-web.

In order to get there, the following tasks have to be completed:

• accept or close all existing pull requests on iris-web (otherwise they are going to be lost)
• create iris-backend and duplicate iris-web, with its history in it (https://docs.github.com/en/repositories/creating-and-managing-repositories/duplicating-a-repository)
• remove everything related to the backend in iris-web (keep the production docker-compose, the nginx Dockerfile and its build on tag)
• remove everything related to the final assembly of the product in iris-backend
• tag a first version of iris-backend (it has to be greater than the latest iris-web version, otherwise there are going to be problems with the existing dockers. Maybe v2.5.0-beta-1)

Some questions/remarks:

• If we are careful about it, it will still be possible to align docker versions with the iris-web release. As an example, it could go like this. Release v2.5.0-beta-1 of iris-backend, integrate in iris-web. Release v2.5.0-beta-1 iris-frontend, integrate in iris-web. Release v2.5.0-beta-2 of iris-backend, integrate in iris-web. Release iris-web v2.5.0-beta-1. Release v2.5.0 of iris-backend. Release v2.5.0 of iris-frontend. Integrate in iris-web. Release v2.5.0 of iris-web.
• We can take the opportunity of this change to simplify the workflow of iris-backend into a single branch workflow. What do you think?
• from the end-user perspective, it will not change anything
• however, for the people who used to propose pull-request in iris-web, they will now have to do the pull-request in the right repository