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/serializers/activitypub/collection_serializer.rb

@@ -13,8 +13,8 @@ class ActivityPub::CollectionSerializer < ActiveModel::Serializer
   attribute :part_of, if: -> { object.part_of.present? }
 
   has_one :first, if: -> { object.first.present? }
-  has_many :items, key: :items, if: -> { (object.items.present? || page?) && !ordered? }
-  has_many :items, key: :ordered_items, if: -> { (object.items.present? || page?) && ordered? }
+  has_many :items, key: :items, if: -> { (!object.items.nil? || page?) && !ordered? }
+  has_many :items, key: :ordered_items, if: -> { (!object.items.nil? || page?) && ordered? }
 
   def type
     if page?