Add filtering for high-volume event types

[I-Dont-Remember]

In the current state, high volume events like message or reaction_added are not as useful as they could be, since we are proxying everything through.

There used to be an Outgoing Webhooks integration with Slack that is a good starting point:

• The Outgoing Webhook will only be triggered when one or both of the following conditions are met:• The message is in the specified Channel
• The message begins with one of the defined Trigger Word(s)

reaction_added should be easy enough to have people submit one to filter on, maybe even down to a small set of channels.
message will be a little harder to get UX right, because (in my head) there are more variety of use cases, could be regex, could be by channel, all sorts of fun crap.

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Ref - Outgoing Webooks Slack app