Make submittable a ListChangesOption

Some scripts rely on the submittable bit from QueryChanges to
implement an 'automatically submit when approved to do so' feature,
to allow a person to mark a change as Autosubmit-Ready and go home
for the night in the expectation that a CI system will verify the
change and cause it to be submitted.

4cd05b2c5e (Only compute submittable
during submitted_together, 2016-09-17), which was needed for
performance reasons when rendering dashboards, broke these scripts.
Allow them to work again through a new o=SUBMITTABLE option.

Change-Id: Ia6ce6d95b1d774ede786b797eb04ce1353de6c86

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/client/ListChangesOption.java

@@ -69,7 +69,10 @@ public enum ListChangesOption {
   PUSH_CERTIFICATES(18),
 
   /** Include change's reviewer updates. */
-  REVIEWER_UPDATES(19);
+  REVIEWER_UPDATES(19),
+
+  /** Set the submittable boolean. */
+  SUBMITTABLE(20);
 
   private final int value;