Decide on essential requirements for GNOME's chat infrastructure

We have a list of all the potential requirements for our chat infrastructure. The next step is to decide which of those requirements are essential.

These requirements will be used to select the shortlist of chat technologies that we will evaluate in detail. The only apply to the evaluation process itself and don't imply an automatic decision to drop our current official chat infrastructure (IRC).

We could extend this exercise to deciding on the priority of other features, but let's focus on what's absolutely essential for now.