Load models from eager_load, not autoload_paths

If something is in `autoload_paths`, it doesn't mean it can or should *always* be loaded. Rails explicitly makes this distinction with `eager_load`.

E.g. if I'm developing a Rails engine, I might want to add `lib` to autoload paths (to ease the development of the engine), but that doesn't mean I want to eagerly load ruby files in `lib/generators/templates`!

Resolves #2770

lib/rails_admin/config.rb

@@ -323,7 +323,7 @@ def viable_models
         included_models.collect(&:to_s).presence || begin
           @@system_models ||= # memoization for tests
             ([Rails.application] + Rails::Engine.subclasses.collect(&:instance)).flat_map do |app|
-              (app.paths['app/models'].to_a + app.config.autoload_paths).collect do |load_path|
+              (app.paths['app/models'].to_a + app.paths.eager_load).collect do |load_path|
                 Dir.glob(app.root.join(load_path)).collect do |load_dir|
                   Dir.glob(load_dir + '/**/*.rb').collect do |filename|
                     # app/models/module/class.rb => module/class.rb => module/class => Module::Class