Send event creation and event update notifications in a background task

The event update notification is made unique so that repeated changes
only trigger one notificate every 30 minutes

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

lib/mobilizon/events/utils.ex

@@ -0,0 +1,18 @@
+defmodule Mobilizon.Events.Utils do
+  @moduledoc """
+  Utils related to events
+  """
+
+  @spec calculate_notification_time(DateTime.t()) :: DateTime.t()
+  def calculate_notification_time(begins_on, options \\ []) do
+    now = Keyword.get(options, :now, DateTime.utc_now())
+    notify_at = DateTime.add(now, 1800)
+
+    # If the event begins in less than half an hour, send the notification right now
+    if DateTime.compare(notify_at, begins_on) == :lt do
+      notify_at
+    else
+      now
+    end
+  end
+end