Change local media attachments to perform heavy processing asynchronously (#13210)

app/controllers/api/v1/media_controller.rb

app/controllers/api/v2/media_controller.rb

app/javascript/mastodon/actions/compose.js

app/models/concerns/attachmentable.rb

app/models/media_attachment.rb

app/serializers/rest/media_attachment_serializer.rb

app/services/post_status_service.rb

app/workers/backup_worker.rb

app/workers/post_process_media_worker.rb

config/application.rb

config/locales/en.yml

config/routes.rb

...ate/20200306035625_add_processing_to_media_attachments.rb

db/schema.rb

lib/paperclip/attachment_extensions.rb