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:

  # 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