Saving state on appliance type

Please review changes and merge if the issue seems resolved
https://redmine.dev.cyfronet.pl/issues/3294
This is very similar to inactive scope. Can you extract common logic into something like:

```ruby
scope :without_vmt_state(state), -> do 
  where(q{
    id NOT IN (
      SELECT appliance_type_id 
        FROM atmosphere_virtual_machine_templates 
          WHERE state = ?
  )}, state) }
```

and then use it in both `inactive` and `not_saving` scopes?

```ruby
scope :inactive, -> { without_vmt_state(:active) }
scope :not_saving, -> { without_vmt_state(:saving) }
```