[BLOCKED] Make `octokit`/`createHookToken` option, create webhook using app and `owner`/`repo` instead

This will simplify the usage of this library.

Instead of using a separate token to create a webhook in a repository or organization, the app can be used itself. However, that requires the `repository_hooks:write` / `organization_hooks:write` permissions, respectively.

First we need to check if the permission is present, based on whether `repo` argument is set or not. What I would do is to retrieve the whole `installation` object instead of just the installation ID, and then check for the permissions key

Here is the current code

https://github.com/gr2m/github-app-webhook-relay/blob/ca79b5405d58e3a566cfdedb14f30a455b6800e9/api/start.js#L47-L51

Then if the correct permission is available, simply instantiate a new octokit instance for that installation like this

```js
const installationOctokit = await state.app.getInstallationOctokit(installation.id);
```

This change will also require updates to the README, stating that neither `createHookToken` nor `octokit` options need to be set if the passed `app` instance has the correct permission to create the repository / organization hook