♻️(backend) move collaboration signal

[AntoLC]

Problem

We signal the collaboration server from the viewset, it is not the best place to do it.

What to do

Better to move it to the model.

Code location

https://github.com/numerique-gouv/impress/blob/7dfc62b2c543064f88be33b7746b8a596633294b/src/backend/core/api/viewsets.py#L525-L526

https://github.com/numerique-gouv/impress/blob/7dfc62b2c543064f88be33b7746b8a596633294b/src/backend/core/api/viewsets.py#L832-L833

https://github.com/numerique-gouv/impress/blob/7dfc62b2c543064f88be33b7746b8a596633294b/src/backend/core/api/viewsets.py#L841-L843