Prepares a safe and clean version of the flash hash for the frontend flash - value of `flash` controller attribute displayable_flash_keys - list of flash keys that will be displayed

Setting cookies for :all domains is broken for Heroku apps, read this article for details devcenter.heroku.com/articles/cookies-and-herokuapp-com

Setting cookies for :all domains also appears to be broken for apps hosted on EC2 behind AWS ELB. No written documentation on that yet, only quantitive analysis based on obvservation of a few instances of load balancers and EC2 instances accessed directly via their IP addresses.

You can also override this method in your controller if you need to customize the cookie domain

List of all flash keys that will be displayed on the frontend. Override this method if you use more flash types.