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/javascript/images/icon_file_download.svg

@@ -0,0 +1,4 @@
+<svg fill="#FFFFFF" height="24" viewBox="0 0 24 24" width="24" xmlns="http://www.w3.org/2000/svg">
+    <path d="M19 9h-4V3H9v6H5l7 7 7-7zM5 18v2h14v-2H5z"/>
+    <path d="M0 0h24v24H0z" fill="none"/>
+</svg>