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>

test/mobilizon/events/utils_test.exs

@@ -0,0 +1,19 @@
+defmodule Mobilizon.Events.UtilsTest do
+  use Mobilizon.DataCase, async: true
+
+  alias Mobilizon.Events.Utils
+
+  @now ~U[2021-11-19T18:17:00Z]
+
+  describe "calculate_notification_time" do
+    test "when the event begins in less than 30 minutes" do
+      begins_on = ~U[2021-11-19T18:27:00Z]
+      assert @now == Utils.calculate_notification_time(begins_on, now: @now)
+    end
+
+    test "when the event begins in more than 30 minutes" do
+      begins_on = ~U[2021-11-19T18:17:00Z]
+      assert begins_on == Utils.calculate_notification_time(begins_on, now: @now)
+    end
+  end
+end