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

Gemfile.lock

@@ -301,6 +301,7 @@ GEM
       mini_mime (>= 0.1.1)
     mario-redis-lock (1.2.0)
       redis (~> 3, >= 3.0.5)
+    memory_profiler (0.9.10)
     method_source (0.9.0)
     microformats (4.0.7)
       json
@@ -664,6 +665,7 @@ DEPENDENCIES
   link_header (~> 0.0)
   lograge (~> 0.7)
   mario-redis-lock (~> 1.2)
+  memory_profiler
   microformats (~> 4.0)
   mime-types (~> 3.1)
   nokogiri (~> 1.8)