Switch RTM to Socket Mode

[tyage]

RTM API is deprecated and Slack recommended Socket Mode
https://api.slack.com/apis/connections/socket

issues:

• Socket mode requires "app-level tokens", so Slack Patron users should manually create an app for each workspace?
• Slack Ruby Client does not support socket mode for now
  • Add support for Slack Socket Mode slack-ruby/slack-ruby-client#414