Refactor notifications code, improve comments, add headers

README.md

notifications/impl.go

notifications/impl_test.go

notifications/legacyRegistration.go

notifications/legacyRegistration_test.go

notifications/ndf.go

notifications/params.go

notifications/providers/apns.go

notifications/providers/fcm.go

notifications/providers/provider.go

notifications/receive.go

notifications/receive_test.go

notifications/registration.go

notifications/registration_test.go

notifications/send.go

notifications/send_test.go

storage/databaseImpl.go