R1:b3397030e6c9

Fix Differential "unsubscribe" so it properly blocks resubscription

Summary:
DifferentialRevision stores this field as a dictionary but the
Editor incorrectly passed it to Herald as a raw value array. Ideally the
property should be called unsubscribedDict or something but I'm increasingly
thinking we're going to centralize subscriptions for Adjutant or some similar
system so I'm disinclined to pursue the schema change just yet. I provided an
explicit raw-value-oriented API, at least.

Test Plan:
With two…