Automatic webhook registration

[smashwilson]

On first launch, test for and register GitHub and DockerHub webhooks.

To do this I'll need:

1. A way to opt in; most likely a variable like HEATLAMP_REGISTER_HOOKS.
2. A list of the repositories and image names to register. Some of these I can infer (HEATLAMP_ANSIBLE_REPO for one) but the DockerHub images will need to be explicitly specified.
3. GitHub credentials to use.
4. A way to specify the externally accessible hostname and port that can receive the hook.
5. Idempotence. Restarting the service shouldn't register a redundant hook, it should just verify that the hooks are registered.
6. Verification of hook presence on the status page, why not.