GitHub App to add webhook

A user should be able to add a module by just pressing a few buttons in the UI, instead of needing to manually add a Webhook:

1. User goes to `https://deno.land/manage`.
2. User clicks on `Sign in with GitHub`
3. User signs in with GitHub
4. User sees list of all their public repositories
5. User selects a repo.
6. User sees a list of modules published from this repo (or empty if none exist).
7. User presses `Add module` button
8. User enters details like name and subdir and presses `Add`.
9. Webhook is added to the repo.