Blind key rotation and stale duration for profiles

See https://blog.dereferenced.org/the-case-for-blind-key-rotation

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

priv/repo/migrations/20200214135231_add_last_refreshed_at_to_actors.exs

@@ -0,0 +1,9 @@
+defmodule Mobilizon.Storage.Repo.Migrations.AddLastRefreshedAtToActors do
+  use Ecto.Migration
+
+  def change do
+    alter table(:actors) do
+      add(:last_refreshed_at, :utc_datetime, null: true)
+    end
+  end
+end