Create S'Ups within a subteam or channel

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Designate a subteam on slack to have S'Up with. Via @jessicabackus

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Related to #30, in #30 (comment) I've described a design for implementing this. Could use help!

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Could start by making channel S'Up explicit.

1. sup #channel would establish a S'Up in a channel, this could also be automatic when bot joins channel
2. sup #channel set size 1 and other options work
3. Instead of enumerating all opted in users, enumerate users in the channel when running a S'Up

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An ongoing rewrite that assumes 1 sup per channel and passes all specs in https://github.com/dblock/slack-sup/tree/channel-sups. Needs to ensure DMing the bot works and migrations.

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One sup per channel is the new default s'up in https://sup2.playplay.io