Add project config boolean to require signed push on a project

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

gerrit-gwtui/src/main/java/com/google/gerrit/client/admin/AdminConstants.java

@@ -43,6 +43,7 @@ public interface AdminConstants extends Constants {
   String useSignedOffBy();
   String createNewChangeForAllNotInTarget();
   String enableSignedPush();
+  String requireSignedPush();
   String requireChangeID();
   String headingMaxObjectSizeLimit();
   String headingGroupOptions();