Re-work the concept of postID to taskID

[yk]

Originally, the frontends were supposed to be stateless, but it turns out more and more that it might be a better path to make the frontends minimally stateful, specially with the way discord works. This means, the entire mapping of postID to taskID that the backend has to do now might be less useful.

The goal of this issue is to conceptually clarify how the protocol would look like if everyone remembers the relevant task IDs (that are initially created for a task, i.e. the ones sent in the ACK messages), what the consequences of that would be and what it would mean in terms of trusting frontends to behave well.

[yk]

Set to low priority. Stateful clients can just do postID = taskID.