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/models/status.rb

@@ -76,7 +76,7 @@ class Status < ApplicationRecord
   scope :not_excluded_by_account, ->(account) { where.not(account_id: account.excluded_from_timeline_account_ids) }
   scope :not_domain_blocked_by_account, ->(account) { account.excluded_from_timeline_domains.blank? ? left_outer_joins(:account) : left_outer_joins(:account).where('accounts.domain IS NULL OR accounts.domain NOT IN (?)', account.excluded_from_timeline_domains) }
 
-  cache_associated :account, :application, :media_attachments, :tags, :stream_entry, mentions: :account, reblog: [:account, :application, :stream_entry, :tags, :media_attachments, mentions: :account], thread: :account
+  cache_associated :account, :application, :media_attachments, :conversation, :tags, :stream_entry, mentions: :account, reblog: [:account, :application, :stream_entry, :tags, :media_attachments, :conversation, mentions: :account], thread: :account
 
   delegate :domain, to: :account, prefix: true