R1:1cd3a593784a

When users resign from revisions, stop expanding projects/packages to include them

Summary:
Depends on D19019. Ref T13053. Fixes T12689. See PHI178.

Currently, if `@alice` resigns from a revision but `#alice-fan-club` is still a subscriber or reviewer, she'll continue to get mail. This is undesirable.

When users are associated with an object but have explicitly disengaged in an individal role (currently, only resign in audit/differential) mark them "unexpandable", so that they can no longer be included…