Add an unique index on posts URLs

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

priv/repo/migrations/20210622133555_add_index_to_posts.exs

@@ -0,0 +1,11 @@
+defmodule Mobilizon.Storage.Repo.Migrations.AddIndexToPosts do
+  use Ecto.Migration
+
+  def up do
+    create_if_not_exists(unique_index("posts", [:url]))
+  end
+
+  def down do
+    drop_if_exists(index("posts", [:url]))
+  end
+end