Ability to remove users from groups

As far as I understand there is no way to remove user from the group after you added them (of course it can be done via console). I have started to implement this functionality and it does work. I had to specify in routes:

```ruby
resources :members, only: [:destroy] do
```

because otherwise the following error appeared in my console: 

```
AbstractController::ActionNotFound - The action 'show' could not be found for Forem::Admin::MembersController:
```

Am I missing some concept here?

I cannot use tests, because running `bundle exec rake forem:dummy_app` gives me an error (will open a separate issue). It would be nice if someone can help me with tests.