When creating a GitHub App, we send a manifest listing the events and

permission the app requires, and we display these to the admin creating

the app. This change includes the events and permissions of any

registered webhook handlers into the manifest and admin display.
We now have public constants for the built in events and permissions

required by core Review Board, making it easy to see and reference these

in docs. This also removes the pull_requests permission from the

defaults since it will be pulled in by Power Pack's pull request

integration handler.
In the future we should probably leverage the "needs attention" GitHub

App state in our admin UI to lets admins know to update their app

settings on GitHub whenever new webhook handlers are registered that

require events/permissions that haven't been granted yet.

Ran unit tests.
Created a GitHub app while having a handler that requires certain

  events and permissions.