[BUG] Incoming messages before the first request has been broadcast are lost

[mattwr18]

Steps to reproduce

1. Start with a clean instance
2. Onboard with a channel, of your choice.
3. After receiving the onboarding message, respond to it.
4. See that the message is completely ignored/lost.

Dev Notes

This occurs because of the logic in how we save messages. At the moment, we require there to be an active_request. This is determined by either the latest request the contributor has received, or the latest request that has been broadcast. This causes multiple issues:

• If there are requests that have been broadcast, but none of them have targeted the contributor, by their tag list, and that contributor responds, it can cause confusion since the message can be assigned to the request that they did not receive.
• It also means that if there are no requests, the messages are simply not saved at all.

There is a proposal to remove the hard requirement that a message belong to a request, see #1649. While this would solve both issues ☝️ , it requires bigger changes.

It has been proposed that we get past the issue where the message is not saved at all by always ensuring that a new instance has a request created with the content of the welcome message. This way, if a contributor replies before the campaign starts it will be assigned to this request.