Improve github-app creation workflow

[rclark]

• 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