Birth dates, birthday reminders API, allow instance admins to require minimum age

Signed-off-by: marcin mikołajczak <git@mkljczk.pl>

lib/pleroma/web/twitter_api/twitter_api.ex

@@ -20,6 +20,7 @@ defmodule Pleroma.Web.TwitterAPI.TwitterAPI do
       |> Map.put(:name, Map.get(params, :fullname, params[:username]))
       |> Map.put(:password_confirmation, params[:password])
       |> Map.put(:registration_reason, params[:reason])
+      |> Map.put(:birth_date, params[:birth_date])
 
     if Pleroma.Config.get([:instance, :registrations_open]) do
       create_user(params, opts)