Use fastlane to post to slack

[nisrulz]

Checkout:

• https://medium.com/ios-development-tips-and-tricks/slack-fastlane-%EF%B8%8F-about-one-of-continuous-integration-puzzles-6ca03576003e
• https://docs.fastlane.tools/actions/slack/
• https://medium.com/@umarmanofpeace/fastlane-the-right-way-with-android-and-slack-cd4ee278f3f4

[stale]

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