Review request approval has historically been represented on

ReviewRequest via a couple of properties (approval and

approval_failure), and by hooks as a tuple or a boolean.
This change moves to a new central definition, ReviewRequestApproval.

This is a typed dictionary that contains a required approval flag and

an optional reason. Reasons can now be given whether approved or not,

allowing for more advanced feedback.
ReviewRequest.get_approval() returns a ReviewRequestApproval, and

the old properties wrap this.
ReviewRequestApprovalHook.is_approved() is soft-deprecated. No

official deprecations at this time, but it has been replaced with a new

get_approval() (which wraps the legacy method if not overridden)

that's better suited for how approval will soon be used.

All unit tests pass.
Built the new hook docs and checked for links and build errors.