Automate Production of TestFlight builds

[aaronbrethorst]

I should be able to use Fastlane or something similar to create TestFlight builds directly from merges to the master branch here on GitHub. I imagine it'll look something like this:

1. A PR is opened.
2. The PR is reviewed and eventually merged to master.
3. The merge to master triggers a GitHub Action that builds and submits the app to Apple for publication on TestFlight.

More info:

• https://byteable.dev/migrate-fastlane-to-use-auth-keys-9113a666cd5d
• https://byteable.dev/build-and-deploy-your-ios-app-with-github-actions-and-fastlane-48c328cc5541