Change GitHub PubSubHubbub API to Hooks API

[ncskier]

We currently use the GitHub PubSubHubbub API to create and delete webhooks.

However, it does not look like GitLab has a PubSubHubbub API, so when we add support for GitLab, we'll need to use the regular hooks API (GitHub hooks API) (GitLab hooks API).

The goal of this issue is to get us ahead of the change by using the GitHub hooks API right now.

Additional info

@dibbles proposed a way to delete a hook without needing the hook id here.