Sync slack messages with the backend

[dennyabrain]

Some tasks that I can think of :

1. Write a SlackMessage module with a handle_message function. See if you can use pattern matching alone to define various handle_message functions for every message type you have identified.
2. Help me come up with a SlackMessage schema. I think useful fields would be text, media_url (when the message is videos, image), conversation_id, channel_id
3. Come up with a SlackChannel schema. I think useful fields would be id, name and organization_id or organization_name

@maanasb01 please copy paste the message payloads you found.