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.

Unit tests pass.