Flash works.

- Extracted FlashController
- Introduced Flash model (with templatePath property)
- Introduced flash helper (with custom ContainerView)
- Updated controllers and application template
Rename createUpcomingBirthday to createUpcomingBirthdayForExistingContact
Extract randomContactName() from randomContact()
Introduce ContactStore#findByName()
Introduce upcomingBirthdayCreation() to reuse existing contacts
Use upcomingBirthdayCreation() on form submission