With the launch of attention set, the UI started requesting
DETAILED_LABELS to populate reviewers on dashboards. Unfortunately,
this also computes which reviewer is allowed to vote on any label.
This check requires getting the group memberships for each reviewer.
This operation is expensive when external group backends are used.
In this change, we expose reviewers also when LABELS are requested.
This will enable the UI to stop requesting DETAILED_LABELS and
request only the cheaper LABELS instead.
Change-Id: I2fb5da919da28c99766109b263124f2cdac5e585
ae92ad8a99