Add default reviewers when finalizing a commit series.

Review Request #10382 — Created Jan. 14, 2019 and submitted — Latest diff uploaded

Review Board

The new DVCS support has a different method for uploading diffs, and the
new support didn't trigger the auto-adding of default reviewers, meaning
that functionality was lost.

This change restores that by adding default reviewers when a commit
series is finalized. To prevent duplicate logic, it also introduces
ReviewRequest.can_add_default_reviewers(), which will check to make
sure that default reviewers are both supported for the review request
and that the no existing diffs have been published.

Unit tests pass.

Manually posted a change using a commit series that previously wasn't
applying default reviewers. Saw that the default reviewers were being
applied successfully.