* Fix BackupService crashing when an attachment is missing

For various reasons such as admin error or out-of-sync media and
database backups, it might be possible for local attachments to be lost.

This commit allows the BackupService to continue its work even if some media
file is missing.

* Change error message

Remove experimental key export

pretty quick fix, and with the 1 week expiration i don't think we need to be too worried about the existing files

closes #6798

* 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