Enable/disable birthday information for auto-generation on server

[oresch]

In the contact app there should be a possibility to enable/disable the birthday information for the server to generate/or not the calendar event.

In my case I got a lot of birthday information in my address book. But I don't want them all to be used for event generation by the server.
Maybe the closed issue 473 and 504 are pointing in the same direction.

My proposal: add a flag to the birthday field (e.g. on/off) which than afterwards shall be taken into account by the server while generating the calendar events.

[MorrisJobke]

This was implemented in 13 by @georgehrke #6884