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/backup.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+# == Schema Information
+#
+# Table name: backups
+#
+#  id                :integer          not null, primary key
+#  user_id           :integer
+#  dump_file_name    :string
+#  dump_content_type :string
+#  dump_file_size    :integer
+#  dump_updated_at   :datetime
+#  processed         :boolean          default(FALSE), not null
+#  created_at        :datetime         not null
+#  updated_at        :datetime         not null
+#
+
+class Backup < ApplicationRecord
+  belongs_to :user, inverse_of: :backups
+
+  has_attached_file :dump
+  do_not_validate_attachment_file_type :dump
+end