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>

priv/repo/migrations/20190929175229_add_rejected_to_participant_role.exs

@@ -0,0 +1,21 @@
+defmodule Mobilizon.Storage.Repo.Migrations.AddRejectedToParticipantRole do
+  use Ecto.Migration
+  alias Mobilizon.Storage.Repo
+  alias Mobilizon.Events.Participant
+  alias Mobilizon.Events.ParticipantRole
+  import Ecto.Query
+
+  @disable_ddl_transaction true
+
+  def up do
+    Ecto.Migration.execute(
+      "ALTER TYPE #{ParticipantRole.type()} ADD VALUE IF NOT EXISTS 'rejected'"
+    )
+  end
+
+  def down do
+    Participant
+    |> where(role: "rejected")
+    |> Repo.delete_all()
+  end
+end