Add a field on Repository for storing a UUID for incoming webhooks.

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.

Reviewed at https://reviews.reviewboard.org/r/6310/