Send email notifications when a participation is approved/rejected

Also handles participant status :rejected instead of deleting the
participation

Closes #164

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

lib/mobilizon_web/schema/event.ex

@@ -111,6 +111,7 @@ defmodule MobilizonWeb.Schema.EventType do
   object :participant_stats do
     field(:approved, :integer, description: "The number of approved participants")
     field(:unapproved, :integer, description: "The number of unapproved participants")
+    field(:rejected, :integer, description: "The number of rejected participants")
   end
 
   object :event_offer do