Populate the group screens via REST

Finish the migration of the groups screen to the new REST API.

Change-Id: If453bd9ff594b6a424fb0fbecde38825ed03808a
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/client/AccountGroup.java

@@ -85,6 +85,11 @@ public final class AccountGroup {
         || uuid.get().matches("^[0-9a-f]{40}$");
   }
 
+  /** @return true if the UUID is for a system group managed within Gerrit. */
+  public static boolean isSystemGroup(AccountGroup.UUID uuid) {
+    return uuid.get().startsWith("global:");
+  }
+
   /** Synthetic key to link to within the database */
   public static class Id extends IntKey<com.google.gwtorm.client.Key<?>> {
     private static final long serialVersionUID = 1L;