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

lib/paperclip/gif_transcoder.rb

@@ -16,7 +16,7 @@ module Paperclip
 
       final_file = Paperclip::Transcoder.make(file, options, attachment)
 
-      attachment.instance.file_file_name    = 'media.mp4'
+      attachment.instance.file_file_name    = File.basename(attachment.instance.file_file_name, '.*') + '.mp4'
       attachment.instance.file_content_type = 'video/mp4'
       attachment.instance.type              = MediaAttachment.types[:gifv]