remove use of Incoming Webhooks in favor of slash command response_url

[aoberoi]

we can get away with less configuration for setting this app up (and a better experience for the installer) if we don't require Incoming Webhooks. the functionality that you use Incoming Webhooks for at the moment could be implemented using the response_url that exists in the slash command payload. See Delayed responses and multiple responses in the docs. i'd be willing to collaborate and contribute.

[aoberoi]

im going to hold off working on this change until i hear if it's something you'd be for merging 😄