Synchronisation between lunch and vacations app

[leo-bogastry]

@dirkjonker gave the idea of synchronising the lunch and vacation apps!
We don't want the user to lose any previous choices, so i think this features has to be user-proactive.

A simple button that allows the user synchronise both apps. If the user is on vacation a certain Friday, then the attendance should be set automatically to "Not attenting". Simple! Simple!

TODO: check with the vacation app people if this can be done