replace ActiveAdmin::Event with ActiveSupport::Notifications

This is the implementation for the discussion in #3364.

`ActiveAdmin::Event` has a MessageQueue implementation that supports the basics, but is not thread safe for example. `ActiveSupport::Notification` provides already a MessageQueue, so why should ActiveAdmin implement his own?

This pull request replace the usage of `ActiveAdmin::Event` with `ActiveSupport::Notifications` and deprecate all `ActiveAdmin::Event` methods, so it can be removed in a future version.

```ruby
ActiveSupport::Notifications.subscribe ActiveAdmin::Application::BeforeLoadEvent do |event, *args|
  # some code
end

ActiveSupport::Notifications.publish ActiveAdmin::Application::BeforeLoadEvent, "some data"
```