Pin chat messages for increased visibility

[Falco]

@j.jaffeux

I think I preferred to start with something subtle and add more emphasis as needed.

I personally prefer the WhatsApp / Telegram model, that is more like ours pin model from the topic list, or even the Reddit model with pins too.

In my opinion it encourages being mindful about pins and cleaning those ASAP, instead of pinning and forgetting those in the pin drawer.