Do not allow to create a room with same participants more than once

As user of REST API,

I want to have only one room for a given participants

Currently, User A and User B chatting each other can have many rooms. For an application, it is time consuming to fetch display a conversation between people when you need to fetch all rooms for these people and merge them.

To simplify this logic, we should only allow to have one room for a given participants. We also want to migrate all existing rooms

This change imply a breaking change of the API behavior AND also a upgrade note about merging all rooms into one for same people

To let people migrate and avoid breaking their app, we should return existing room on POST (instead of blocking them). But another related ticket should be done in some months to remove this logic and return 409 error