Support using IRC as fallback

[m4-used-rollout]

Twitch seems to have occasional issues with EventSub. It would be nice if, in case of another EventSub outage, we could throw a switch in the configuration and have the core use the old IRC library we abandoned in favor of EventSub to ingest messages.

[m4-used-rollout]

This may also function as a temporary workaround for #428 assuming the code that parses sub messages from IRC chat still works.

[Felk]

This is referring to the irc code that was removed in #415
The concern is that EventSub has more outages than IRC and we could increase our uptime if we could fall back to IRC. At the cost of maintaining both ways.