A GitHub App has a single, app-wide webhook URL that receives events from

every account the app is installed on. We have the GitHubAppWebhookView for

this, which currently only handles app installation events. We should have an

easy way for us and extension authors to handle GitHub webhook events too.
This change introduces the BaseGitHubAppWebhookHandler, which can be

subclassed to list the events the handler supports and the required GitHub App

permissions. Events will be passed to handlers as a GitHubAppWebhookEvent,

containing the event's raw payload, delivery ID, and the account(s) and

repository it may apply to. The actual dispatching to handlers will come in

a later change.
This also adds a registry for the handlers. The registry has a helper for

merging all the events and required pemissions of the registered handlers,

so that they can all be shown and requested when creating a GitHub App. The

hook for registering handlers will come in a later change.

Ran unit tests.
Used in the Pull Request integration work.