Add extra_data fields and API support to a number of objects.

This adds extra_data fields to DiffSet, DiffSetHistory, FileDiff,
ReviewRequest, Group, Review, and ReviewRequestDraft. Along with this,
there's API support for retrieving and modifying extra_data for DiffSet,
FileDiff, ReviewRequest, Group, Review, and ReviewRequestDraft.

This change may impact upgrade times for larger installs, but is
necessary for future expansion.