Abstract out two slack-related functions

Abstract out two slack-related functions

The two new functions are slack_is_valid_request and slack_send_reply.
These functions provide some shared code for dealing with slack to
avoid code duplication.

These functions also provide a debug mode (that doesn't use slack at
all). This is a convenience for testing the implementation of slash
commands in a local "make run" session without needing to touch Slack
endpoints at all. (Instead, all requests are considered valid and
slack replies are simply printed to stdout.)