Add privateByDefault config to /config/server/info

Exposing 'privateByDefault' there would be supper useful for polygerrit
when we are implementing the create change dialog.

Change-Id: I248210124a2d972934a94c42c32ccae39c0cfca3

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/common/ChangeConfigInfo.java

@@ -19,6 +19,7 @@ public class ChangeConfigInfo {
   public Boolean showAssigneeInChangesTable;
   public Boolean allowDrafts;
   public int largeChange;
+  public Boolean privateByDefault;
   public String replyLabel;
   public String replyTooltip;
   public int updateDelay;