Implement realtime updates of messages with client via channels

[begedin]

Problem

From #1234 (comment)

The idea behind the messaging feature is that it ought to work mostly in realtime, while still supporting async communication.

In order to achieve that, we need to be able to update both sides of the conversation about any changes to it in real time.

The most obvious approach to achieve that would be the phoenix channel feature.

[joshsmith]

Closed by #1301