This is controlled by receive.requireSignedPush in the project.config,
which is a separate bit from receive.enableSignedPush. (Adding an
inheritable tri-state enum would have been complex to implement and
have hard-to-define semantics.)
requireSignedPush is only inspected if enableSignedPush is true; this
allows project owners to temporarily disable signed push entirely e.g.
due to a bug, without having to flip both bits.
Change-Id: I07999b6fa185d470b30509941473e3158f9dfa2c
0543c735eb