Channel configuration should be stored in database

[cpruitt]

Currently making a channel config change requires editing config/matchmaking.yml. We'd like to make it possible to use slash commands but yaml prevents that. Moving config into the database is a first step in working on that command.

Proposed Solution (Not tied to this / Open to better options)

Perhaps we consider creating a SlackChannel model that can be looked up by either Channel name or Slack Channel ID. (whichever is better). That model can have a #config attribute stored in the DB as a JSON column. We would use this in place of the current matchmaking initialization's use of YAML.