Add config to disable private changes

This commit adds a config option so that we can disable
private changes. With this config enabled, users will not
be able to create private changes through UI or Git push.
Users can't set a change to be private through the
PostPrivate REST endpoint, either.

Change-Id: I2794c140f37bd3940b0a2fad68abbced1145c843

java/com/google/gerrit/extensions/common/ChangeConfigInfo.java

@@ -18,6 +18,7 @@ public class ChangeConfigInfo {
   public Boolean allowBlame;
   public Boolean showAssigneeInChangesTable;
   public Boolean allowDrafts;
+  public Boolean disablePrivateChanges;
   public int largeChange;
   public String replyLabel;
   public String replyTooltip;