config: investigate github app import

[alejandromumo]

          question: importing the `REMOTE_APP` could be troublesome if it was replaced by another instance. Perhaps we should get the github remote app using the oauth-client extension.

E.g:

remote = LocalProxy(
        lambda: current_oauthclient.oauth.remote_apps[
            current_app.config["GITHUB_WEBHOOK_RECEIVER_ID"]
        ]
    )

However this is config, can we lazy load?

Originally posted by @alejandromumo in #99 (comment)