Fix routes generator for Rails 6 zeitwerk autoloader

Created by: excid3

The classic autoloader worked fine for looking up all models when running the routes generator.

Rails 6 introduces zeitwerk as the default autoloader and it does not load all the models until needed.

I did some inspection to see why Administrate was only loading only my Devise model and nothing else.

> ActiveRecord::Base.descendants
=> [ApplicationRecord(abstract), User (call 'User.connection' to establish a connection)]
> Rails.autoloaders.main.eager_load
=> true
> ActiveRecord::Base.descendants
=> [ApplicationRecord(abstract), User (call 'User.connection' to establish a connection), Announcement (call 'Announcement.connection' to establish a connection), Notification (call 'Notification.connection' to establish a connection), Service (call 'Service.connection' to establish a connection)]

After the eager load, all the models are available. In the old / classic autoloader from Rails 5 and earlier, you'd get the second set of results the first time.

This fixes it so Rails 6 will generate all the models like it used to.