Attach event ics files to event-related emails

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

mix.exs

@@ -133,6 +133,7 @@ defmodule Mobilizon.Mixfile do
       {:xml_builder, "~> 2.1.1", override: true},
       {:remote_ip, "~> 0.2.0"},
       {:ex_cldr_languages, "~> 0.2.1"},
+      {:slugger, "~> 0.3"},
       # Dev and test dependencies
       {:phoenix_live_reload, "~> 1.2", only: [:dev, :e2e]},
       {:ex_machina, "~> 2.3", only: [:dev, :test]},