Merge branch 'download_link' into 'master'

Gemfile

app/controllers/concerns/downloadable_controller.rb

app/controllers/v1/collections_controller.rb

app/controllers/v1/learning_objects_controller.rb

app/models/collection.rb

app/models/concerns/downloadable.rb

app/models/concerns/sociable.rb

app/models/download.rb

app/models/learning_object.rb

app/models/package.rb

app/models/package_item.rb

app/serializers/learning_object_serializer.rb

app/services/package_service.rb

app/services/package_service/generator.rb

app/services/package_service/link.rb

app/workers/package_worker.rb

config/environments/development.rb

config/routes.rb

db/migrate/20170215113455_add_ip_to_downloads.rb

db/migrate/20170215121042_add_index_to_downloads.rb