feat: silence slack messages during quiet period

[erickzhao]

Doesn't post to Slack during December.

For individual label additions, I think this change depends on Octokit's pull_request.updated_at prop to also reflect the latest time a label was added, since webhook payloads don't seem to come with a timestamp. (Alternatively, we could check the default new Date() constructor call similar to how it's done for the reminders.) This now uses new Date() for everything.

[samuelmaddock]

This is the message for newly posted APIs, right? I'd personally like to still see the messages since they only get posted when they're initially created.

I think a compromise could be:

• Don't @ the wg-api team during December
• Disable the reminder listing all open PRs during December

[erickzhao]

This is the message for newly posted APIs, right? I'd personally like to still see the messages since they only get posted when they're initially created.

@samuelmaddock Currently it's both of them! I can disable the ping only as a compromise and keep new PR messages coming in :)