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/controllers/settings/exports_controller.rb

@@ -1,11 +1,23 @@
 # frozen_string_literal: true
 
 class Settings::ExportsController < ApplicationController
+  include Authorization
+
   layout 'admin'
 
   before_action :authenticate_user!
 
   def show
-    @export = Export.new(current_account)
+    @export  = Export.new(current_account)
+    @backups = current_user.backups
+  end
+
+  def create
+    authorize :backup, :create?
+
+    backup = current_user.backups.create!
+    BackupWorker.perform_async(backup.id)
+
+    redirect_to settings_export_path
   end
 end