Move ActiveAdmin Route Generation Into Engine

ActiveAdmin now internalizes all its route handling. This means you no longer need to manually hook ActiveAdmin into your routes.

This allows for cleaner integration between ActiveAdmin and your application, and also allows you to exclude it on a per-environment basis by including it only in certain groups. Example:

```ruby
# Gemfile
group :admin
  gem 'active_admin'
end

# config/application.rb
groups = {
  assets: %w[development test],
  admin: %w[production staging],
}

if defined?(Bundler)
  Bundler.require *Rails.groups(groups)
end
```

Resolves #2126