Fix issue when updating event and introduce background jobs

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

priv/repo/migrations/20191031142145_add_unique_index_on_actor_and_event_for_participant.exs

@@ -0,0 +1,7 @@
+defmodule Mobilizon.Storage.Repo.Migrations.AddUniqueIndexOnActorAndEventForParticipant do
+  use Ecto.Migration
+
+  def change do
+    create(unique_index(:participants, [:event_id, :actor_id]))
+  end
+end

priv/repo/migrations/20191104093447_add_oban_jobs_table.exs

@@ -0,0 +1,13 @@
+defmodule Mobilizon.Storage.Repo.Migrations.AddObanJobsTable do
+  use Ecto.Migration
+
+  def up do
+    Oban.Migrations.up()
+  end
+
+  # We specify `version: 1` in `down`, ensuring that we'll roll all the way back down if
+  # necessary, regardless of which version we've migrated `up` to.
+  def down do
+    Oban.Migrations.down(version: 1)
+  end
+end