Add Push notifications backend support

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

priv/repo/migrations/20210506080303_create_user_push_subscriptions.exs

@@ -0,0 +1,16 @@
+defmodule Mobilizon.Repo.Migrations.CreateUserPushSubscriptions do
+  use Ecto.Migration
+
+  def change do
+    create table(:user_push_subscriptions, primary_key: false) do
+      add(:id, :uuid, primary_key: true)
+      add(:user_id, references(:users, on_delete: :nothing), null: false)
+      add(:digest, :text, null: false)
+      add(:data, :map, null: false)
+
+      timestamps()
+    end
+
+    create(unique_index(:user_push_subscriptions, [:user_id, :digest]))
+  end
+end