Use Socket Mode instead of RTM API

Goal

New Slack apps may not use any Real-Time Messaging API method. So that Joe Bot has to connect to Slack using Socket Mode: https://api.slack.com/apis/connections/socket-implement

The official library supports Socket Mode since v0.8.0 https://github.com/slack-go/slack/pull/885

TODO / How to implement

Upgrade the official library - https://github.com/slack-go/slack
Implement support for a new communication type - Socket Mode
Update the docs and config examples

Acceptance criteria

Edited Jun 15, 2021 by Artyom Kartasov