Account archive download (#6460)

* Fix #201: Account archive download

* Export actor and private key in the archive

* Optimize BackupService

- Add conversation to cached associations of status, because
  somehow it was forgotten and is source of N+1 queries
- Explicitly call GC between batches of records being fetched
  (Model class allocations are the worst offender)
- Stream media files into the tar in 1MB chunks
  (Do not allocate media file (up to 8MB) as string into memory)
- Use #bytesize instead of #size to calculate file size for JSON
  (Fix FileOverflow error)
- Segment media into subfolders by status ID because apparently
  GIF-to-MP4 media are all named "media.mp4" for some reason

* Keep uniquely generated filename in Paperclip::GifTranscoder

* Ensure dumped files do not overwrite each other by maintaing directory partitions

* Give tar archives a good name

* Add scheduler to remove week-old backups

* Fix code style issue

app/mailers/user_mailer.rb

@@ -66,4 +66,16 @@ class UserMailer < Devise::Mailer
       mail to: @resource.email, subject: I18n.t('user_mailer.welcome.subject')
     end
   end
+
+  def backup_ready(user, backup)
+    @resource = user
+    @instance = Rails.configuration.x.local_domain
+    @backup   = backup
+
+    return if @resource.disabled?
+
+    I18n.with_locale(@resource.locale || I18n.default_locale) do
+      mail to: @resource.email, subject: I18n.t('user_mailer.backup_ready.subject')
+    end
+  end
 end