Factor out common parts of ReviewRequest and ReviewRequestDraft.

ReviewRequest and ReviewRequestDraft have always had many fields and
methods that operated the same way but were defined separately. This has
led to a lot of duplicate code, which has always been a pain to keep
updated.

Now these common fields and methods have been split out into a new
base class, BaseReviewRequestDetails. This covers the summary,
description, testing done, bugs closed, branch, field normalization
during save, human-readable bug list, screenshot/file attachment
accessors, default reviewers, and updating from a changenum.

General playing around with the review requests seems to work.

All unit tests pass.