Allow to duplicate 'Batch Actions' button at the bottom

Switches 'Batch Actions' button selector from ID-based to class-based which allows to use multiple (duplicated) batch buttons. Duplicating button at the bottom speeds up batch operations on large tables (user don't have to scroll back to the top).

The following monkey-patch put into `app/config/initializers/active_admin.rb` does the trick:

```ruby
module ActiveAdmin
  module Views
    module Pages
      class Index
        def main_content
          wrap_with_batch_action_form do
            build_table_tools
            build_collection
            build_table_tools
          end
        end
      end
    end
  end
end
```

`e.preventDefault()` is needed to prevent browser's default behavior when clicking on link with `href='#'` - it scrolls to the top