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

config/routes.rb

@@ -76,7 +76,7 @@ Rails.application.routes.draw do
     resource :notifications, only: [:show, :update]
     resource :import, only: [:show, :create]
 
-    resource :export, only: [:show]
+    resource :export, only: [:show, :create]
     namespace :exports, constraints: { format: :csv } do
       resources :follows, only: :index, controller: :following_accounts
       resources :blocks, only: :index, controller: :blocked_accounts