Separate CCs from reviewers when NoteDb is enabled

When the server config reports `note_db_enabled` is true, we have the
opportunity to display reviewers by state. There are just two states
we care about: `REVIEWER` and `CC`. Reviewers in the `REVIEWER` state
will continue to be managed in the existing "Reviewers" fields in
<gr-change-metadata> and <gr-reply-dialog>. We need similar but separate
"CC" fields in the same elements for managing reviewers in the `CC`
state.

Because the reply dialog may now have two editable account lists, the
logic for tracking reviewer state and filtering autocomplete suggestions
needs to move to <gr-reply-dialog>. In particular, we don't want pending
additions in one field to show up as suggestions in the other field. We
also need to make sure that the large group confirmation overlay can
handle confirmations for either field, and return focus to the right
element when closed.

Feature: Issue 3810
Change-Id: Id5111220906ad728707087f6fb2ddb085c34a9b8