This adds new Repository.hooks_uuid, which is used for matching and

validating incoming webhooks. How this is used is up to the hook

implementation, and can be part of the URL, part of HMAC validation,

or whatever else makes sense.
This field is not exposed in the API. It will instead be used by the

various HostingServices in whichever way makes the most sense for their

implementation, and exposed to the user through an upcoming change that

will allow them to provide hook instructions.

Made use of the field and functions in another change.