Fix request verification in Slack event handler

[gigxz]

Incoming events from Slack should be verified using the signing secret. I couldn't get it working so it's disabled right now:

gateway/metagov/metagov/plugins/slack/handlers.py

Lines 299 to 309 in 0aaf7bc

	def verify_signature(request, timestamp, signature):
	# FIXME! this isn't working for some reason
	return True
	signing_secret = SIGNING_SECRET
	signing_secret = bytes(signing_secret, "utf-8")
	body = request.body.decode("utf-8")
	base = f"v0:{timestamp}:{body}".encode("utf-8")
	request_hash = hmac.new(signing_secret, base, hashlib.sha256).hexdigest()
	expected_signature = f"v0={request_hash}"
	return hmac.compare_digest(signature, expected_signature)