Add fields to ChangeInfo necessary for ApprovalTable

Includes the mergeability flag and the list of reviewers that may be
removed by the current user, the latter only when DETAILED_LABELS is
set. Like permitted_labels, this is a top-level, current-user-specific
property.

Change-Id: I7c8fae4c1cde8cb138d31fce587a327f145bf175