Backend: Support cc

Support for cc in:
* Config
* REST
* Adding ccs on patchset upload

Maintain backwards compatibility by defaulting the newly introduced
ConfigReviewersInput#type to "REVIEWERS".

TODO:
Frontend support for cc.

Feature: Issue 11676
Change-Id: I2c2aba662f1a45ed4f2540b7eb32ee56cc8faaba
12 files changed