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

db/migrate/20180211015820_create_backups.rb

@@ -0,0 +1,11 @@
+class CreateBackups < ActiveRecord::Migration[5.1]
+  def change
+    create_table :backups do |t|
+      t.references :user, foreign_key: { on_delete: :nullify }
+      t.attachment :dump
+      t.boolean :processed, null: false, default: false
+
+      t.timestamps
+    end
+  end
+end