Base group index on new class InternalGroup

To be able to add the members and sub-groups of a group to the group
index, we need a class which contains those additional details. For
this reason, we introduce a new class InternalGroup and base the
group index on it (instead of AccountGroup).

Change-Id: I88b1a0bf1bd06804069a2a3165e7664db7900903

gerrit-common/src/main/java/com/google/gerrit/common/data/GroupDescription.java

@@ -48,6 +48,7 @@ public class GroupDescription {
 
     AccountGroup.Id getId();
 
+    @Nullable
     String getDescription();
 
     AccountGroup.UUID getOwnerGroupUUID();

gerrit-common/src/main/java/com/google/gerrit/common/data/GroupDescriptions.java

@@ -51,6 +51,7 @@ public class GroupDescriptions {
       }
 
       @Override
+      @Nullable
       public String getDescription() {
         return group.getDescription();
       }