Create management command for onboarding an existing app

[joshuadavidthomas]

Right now an Installation is only created from the "installation.created" webhook event. If someone wants to use this library with an app that already exists, they won't ever get this event so the installation will never be created.

There should be a way to onboard an existing application and installation -- my first thought was a management command, but open to other solutions.

[joshuadavidthomas]

I suppose someone could theoretically uninstall and reinstall the application, but I'm not sure I like that because I'm not totally sure of the knock-on effects. You would lose the history of webhook deliveries for one, and you would probably have to go through authorizating the new app for all repos and events again.