Add helpers for flash messages

This is a pattern that I found worked really well on a previous project. I mentioned it to @jessieay who also used it on a recent package and enjoyed it.

This moves the idea of what should go into the flash out of the controller and completely into i18n.

I considered this as a separate gem but it seems like a lot of overhead for a few methods. I also imagine that people would want to modify these methods in their own application after the original creation of the app.