Investigate EventSub whispers

[m4-used-rollout]

According to community reports, as of October 13, the bot is suddenly no longer reacting to whispers. It is still able to send them. Please investigate whether EventSub changed how whispers work.

[Felk]

So far: it appears we're sometimes getting a Bad Request for our EventSub subscription call. TwitchLib only gives us a vague reason and throws the actual reason from twitch away: https://github.com/TwitchLib/TwitchLib.Api/blob/2ea61c70225c0c15d7def7a5808837191e33d778/TwitchLib.Api.Core/HttpCallHandlers/TwitchWebRequest.cs#L124

The EventSub request doesn't get retried because Bad Request currently isn't listed as retryable. Maybe it is?

[Felk]

seems to have been fixed by #442

The bad requests' actual message was that a websocket connection cannot have subscriptions of different users.