Create two magical groups 'Anonymous Users' and 'Registered Users'

These groups represent everyone on Gerrit.  Anonymous users is used for
a user account when they are not yet signed in to the site.  Registered
users are used for any account which is signed in.

Like the admin group, these groups are blessed by being marked in the
system config table as the special groups.  Since their membership is
managed automatically by Gerrit we do not permit a group owner to edit
the membership list in the web UI.  We also don't show the membership,
as the membership isn't stored in the database, its computed on the fly
under the assumption that everyone is in either group if they have an
Account.Id assigned.

Signed-off-by: Shawn O. Pearce <sop@google.com>

appjar/src/main/java/com/google/gerrit/client/reviewdb/SystemConfig.java

@@ -91,6 +91,14 @@ public final class SystemConfig {
   @Column
   public AccountGroup.Id adminGroupId;
 
+  /** Identity of the anonymous group, which permits anyone. */
+  @Column
+  public AccountGroup.Id anonymousGroupId;
+
+  /** Identity of the registered users group, which permits anyone. */
+  @Column
+  public AccountGroup.Id registeredGroupId;
+
   protected SystemConfig() {
   }
 }