Add birthday field to user profile data and include notification for followers

[brootle]

This was suggested by AgustinOlmedo and I think this is great idea. It's pretty simple, add one more field to user profile to input birthday and send notifications about coming birthday to followers. I believe this must be basic feature for social protocol.

cc @tijno

[ItsAditya-xyz]

lol @lazynina is already working on profile extra data which will solve all this

[brootle]

basic profile features like that should be stored as simple profile field, if you don't believe me, go ask my friend dharmesh