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 accounts, A and B: created universal Herald CC rule with
user B, created a revision with user A, had user B unsubscribe, had user A
update the revision. User B was not resubscribed.

Reviewed By: aran
Reviewers: jungejason, tuomaspelkonen, aran
CC: ola, aran, epriestley
Differential Revision: 188