Add instance option to append a CSS class for rows

[ghost]

Follow up of PR #2606 , with tests

As Rails Admin uses Bootstrap, I wanted to use the table's contextual classes in my lists.
This is an approach to implement that using a method declared in RailsAdmin::Config,
it has access to the object being rendered into the row.

Usage

RailsAdmin.config do |config|
config.model Event do
  list do
    row_css_class do
      bindings[:object].mode == 'panic' ? 'danger' : 'success'
    end
end

[mshibuya]

Thanks 👍