Configure signed push verification on a per-project basis

Add a new inheritable boolean "enableSignedPush" to configure the
hook for each project, assuming it is enabled for the server. For the
default case, where it is not configured on the server, do not
surface this value in the UI at all.

Change-Id: I51baab5bfc15607a3c47ee8752b10d4972e8550e
diff --git a/Documentation/rest-api-projects.txt b/Documentation/rest-api-projects.txt
index a25c7bb..4658e2c 100644
--- a/Documentation/rest-api-projects.txt
+++ b/Documentation/rest-api-projects.txt
@@ -731,6 +731,7 @@
     "use_content_merge": "INHERIT",
     "use_signed_off_by": "INHERIT",
     "create_new_change_for_all_not_in_target": "INHERIT",
+    "enable_signed_push": "INHERIT",
     "require_change_id": "TRUE",
     "max_object_size_limit": "10m",
     "submit_type": "REBASE_IF_NECESSARY",
@@ -774,6 +775,11 @@
       "configured_value": "TRUE",
       "inherited_value": true
     },
+    "enable_signed_push": {
+      "value": true,
+      "configured_value": "INHERIT",
+      "inherited_value": false
+    },
     "max_object_size_limit": {
       "value": "10m",
       "configured_value": "10m",
@@ -1902,6 +1908,9 @@
 valid link:user-changeid.html[Change-Id] footer in any commit uploaded
 for review is required. This does not apply to commits pushed directly
 to a branch or tag.
+|`enable_signed_push`                      |optional|
+link:#inherited-boolean-info[InheritedBooleanInfo] that tells whether
+signed push validation is enabled on the project.
 |`max_object_size_limit`     ||
 The link:config-gerrit.html#receive.maxObjectSizeLimit[max object size
 limit] of this project as a link:#max-object-size-limit-info[