Track usage of media files and add a job to clean them

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

lib/mobilizon/posts/posts.ex

@@ -103,7 +103,7 @@ defmodule Mobilizon.Posts do
   @spec update_post(Post.t(), map) :: {:ok, Post.t()} | {:error, Ecto.Changeset.t()}
   def update_post(%Post{} = post, attrs) do
     post
-    |> Repo.preload(:tags)
+    |> Repo.preload([:tags, :media])
     |> Post.changeset(attrs)
     |> Repo.update()
   end