IRC setup tool: channel topic should include timezone information

[tidoust]

The tool that initializes IRC channels includes the time of the breakout at the end of the channel topic. For breakout days, it's not obvious that this time is to be interpreted as UTC. For TPAC breakouts, it's relatively obvious that this time is expressed in the timezone of the place where TPAC takes place, but then it remains a good idea to be explicit about that.

One hiccup to address is that we only have a timezone identifier in the project's metadata, and that identifier isn't necessarily a human friendly timezone label (e.g., the metadata needs to be "Etc/UTC" for a UTC based event).