Save user locale and use it to translate things

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

priv/repo/migrations/20191001074442_add_locale_to_users.exs

@@ -0,0 +1,9 @@
+defmodule Mobilizon.Storage.Repo.Migrations.AddLocaleToUsers do
+  use Ecto.Migration
+
+  def change do
+    alter table(:users) do
+      add(:locale, :string, default: "en")
+    end
+  end
+end