Make Channel Configurable

[ghickman]

As part of #110 we would have preferred to emit messages to not-#general. However, to do that in the webhook handler we would have had to let it know more directly about the webhooks/projects it handles or couple it to job configs.

The preferable solution is to make channel a configurable option for a job and have a scheduled task look that up when it gets run, rather than storing it in the job table.