Replace custom flash implementation with ember-flash-message
Display empty list flash when already set up

The `birthdayListIsEmpty` flash was displayed in `setupController()`.
But it seems it was not called anymore when controller was already set up.
Moving it to the route's `didTransition` action partially fixes this
issue.
Flash is still not rendered when directly accessing /birthdays, while
redirection works, so this may be related to ember-flash-message not
rendering flashes on startup, I'm not really sure...