Speed up some rake tasks by moving execution to Sidekiq (#7678)

* Speed up some rake tasks by moving execution to Sidekiq

mastodon:media:remove_silenced
mastodon:media:remove_remote
mastodon:media:redownload_avatars
mastodon:feeds:build

* Fix code style issue

app/workers/maintenance/redownload_account_media_worker.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class Maintenance::RedownloadAccountMediaWorker
+  include Sidekiq::Worker
+
+  sidekiq_options queue: 'pull', retry: false
+
+  def perform(account_id)
+    account = Account.find(account_id)
+    account.reset_avatar!
+    account.reset_header!
+    account.save
+  rescue ActiveRecord::RecordNotFound
+    true
+  end
+end