Introduce comments below events

Also add tomstones

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

priv/repo/migrations/20191127163737_create_tombstones.exs

@@ -0,0 +1,14 @@
+defmodule Mobilizon.Repo.Migrations.CreateTombstones do
+  use Ecto.Migration
+
+  def change do
+    create table(:tombstones) do
+      add(:uri, :string)
+      add(:actor_id, references(:actors, on_delete: :delete_all))
+
+      timestamps()
+    end
+
+    create(unique_index(:tombstones, [:uri]))
+  end
+end