Create a new Slack channel when creating a hunt

Create a new Slack channel when creating a hunt

Using the slug of the hunt for the channel name.

Some thoughts on improving this:

  * Should we modify the slug in some way to distinguish this as a
    hunt channel?

  * We should definitely do validation of the legality of the slug for
    a channel name.

  * We should invite the triggering user to the channel

  * We should put the hunt URL into the topic or description

  * Do we want to create a sheet for hunt channels or only for puzzle
    channels?