Slack integration v2

[dominik-hadl]

Slack integration should be expanded to provide better information. Slack has recently released the Block Kit interface for building/sending messages through their API which we should explore and see how it could be appended.

https://api.slack.com/block-kit

Checklist of ideas that the new messages could contain:

• [ ] App icon (useful when badged)
• [ ] Buttons that link to Bitrise, HockeyApp
• [ ] Changelog (maybe expandable, so the message isn't taking up whole Slack window)
• [ ] Build author, timestamp
• [ ] Testflight link/status (this is admittedly the most difficult, maybe a microservice would be needed to monitor this)
• [ ] Retrigger button on failed builds?
• [ ] more....