Fix issue when updating event and introduce background jobs

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

mix.exs

@@ -100,6 +100,7 @@ defmodule Mobilizon.Mixfile do
       {:ex_cldr_dates_times, "~> 2.0"},
       {:ex_optimizer, "~> 0.1"},
       {:progress_bar, "~> 2.0"},
+      {:oban, "~> 0.10"},
       # Dev and test dependencies
       {:phoenix_live_reload, "~> 1.2", only: [:dev, :e2e]},
       {:ex_machina, "~> 2.3", only: [:dev, :test]},