Support "!unsubscribe" in Differential reply handler

Summary:
See task. Allows users to unsubscribe via email.

Test Plan:
Used mail receiver to unsubscribe from a revision. Tested subscribe/unsubscribe
buttons. Verified "!unsubscribe" appears as an avilable action in email.

Reviewed By: ola
Reviewers: ola, mroch, jungejason, tuomaspelkonen, aran
CC: aran, epriestley, ola
Differential Revision: 385