feat(slack): add channel reaction feedback #273

Job	Run time
docker-build (api, api/Dockerfile)	1m 18s
docker-build (integrations-slack-gateway, integrations/slack-gateway/Dockerfile)	23s
test (integrations-slack-gateway, integrations/slack-gateway)	21s
test (integrations-github-app, integrations/github-app)	15s
test (api, api)	18s
test (operator, operator)	21s
test (acptext, integrations/acptext)	14s
	3m 10s

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