User Groups. Move from MVP to MVVM pattern

Since we decided to move from MVP based on Views and Presenters to MVVM model with @command, @bind annotations, we need to finish this and do the same with User Group page.
So in package org.jtalks.poulpe.web.controller.group there are couple of classes left that have View and Presenter in their names - this is what should be replaced with ViewModel (UserGroupVm is the preferred name for this class).
Take a look at other implementations of Vm files in order to understand how it works and google for ZK MVVM for more details.
Acceptance criteria:

1. Everything works as it was working before.
2. Everything is covered with unit tests.