Default follows for new users (#4871)

When a new user confirms their e-mail, bootstrap their home timeline
by automatically following a set of accounts. By default, all local
admin accounts (that are unlocked). Can be customized by new admin
setting (comma-separated usernames, local and unlocked only)

app/workers/bootstrap_timeline_worker.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class BootstrapTimelineWorker
+  include Sidekiq::Worker
+
+  def perform(account_id)
+    BootstrapTimelineService.new.call(Account.find(account_id))
+  end
+end