Thread unsafe lazy initialization in PostActionType

[eregon (Benoit Daloze)]

It could also be done like this, that would notably avoid reading the ivar twice and make it clearer it’s eager-initialized (the logic could also be in a initial_flag_settings helper method for organization of course):

  class << self
    attr_reader :flag_settings
  end

  @flag_settings = FlagSettings.new
  @flag_settings.add(
    3,
    :off_topic,
    notify_type: true,
    auto_action_type: true,
  )
  # ...