Configurable post_new hook for the Channel model

[brizandrew]

If we're going to get events_handler working, we'll need a configurable way to hit that API on creation. May just be worth adding it as a payload type to hit all the webhooks.

[brizandrew]

It would suck thought to have all new messages hit this endpoint. Maybe we should introduce boolean fields for the Webhook model that indicates which types of updates they should be sent. This way we could keep this to "new_channel" payloads.