270 batch edit api

A small update which only displays the destroy action by default if it has been assigned for that resource.

This fixes a problem we found when testing out batch actions with an existing app which had some views that restricted the available actions, eg:

```ruby
ActiveAdmin.register Model do
  actions :index
  # or
  #actions :all, :except => [:destroy]
end
```

Aside from that, excellent work on this branch!