Add a config option to whitelist users registration

Through whole email or domain email

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/mobilizon_web/schema/config.ex

@@ -13,6 +13,7 @@ defmodule MobilizonWeb.Schema.ConfigType do
     field(:description, :string)
 
     field(:registrations_open, :boolean)
+    field(:registrations_whitelist, :boolean)
     field(:demo_mode, :boolean)
     field(:country_code, :string)
     field(:location, :lonlat)