refactorings and small TODOs

[ghost]

• [ ] We keep specifying the top-level text over and over, with small variations. We should pull that out.
• [ ] Log the event's basic info (request id, PR number, etc) at the start of execution
• [ ] replace that stupid dispatcher with just await this["handle_${event.action}"](channel, event) (with safety checks if it's not found)
• [ ] Have onPrThread take as an arg a new SlackIO, which automatically adds the thread_id. I keep getting bitten by forgetting it in that context!
• [x] Look for other opportunities to clean up, because there's a bunch of cruft in there already.

[ghost]

Look for other opportunities to clean up, because there's a bunch of cruft in there already.

I'm marking this as done without action, since it's not really actionable.