Improve github-app creation workflow

Add more instruction here about what will happen next (i.e. redirect to GitHub to create the app)
After creating app and receiving credentials here, user should be given instructions for installing the GitHub app, getting an installation id, and provide the program with it via form.
Only once we have an installation id should secrets be saved
The app's webhook secret needs to be conveyed to the user, or the event-ingestion function should look it up in secrets manager instead of getting it via environment.
[bonus] Allow user to select a set of events to forward

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