Fix posts not able to accept existing tags

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

lib/mobilizon/posts/post.ex

@@ -22,8 +22,8 @@ defmodule Mobilizon.Posts.Post do
   import Ecto.Changeset
   alias Ecto.Changeset
   alias Mobilizon.Actors.Actor
+  alias Mobilizon.{Events, Medias}
   alias Mobilizon.Events.Tag
-  alias Mobilizon.Medias
   alias Mobilizon.Medias.Media
   alias Mobilizon.Posts.Post.TitleSlug
   alias Mobilizon.Posts.PostVisibility
@@ -141,6 +141,12 @@ defmodule Mobilizon.Posts.Post do
 
   defp put_tags(changeset, _), do: changeset
 
+  @spec process_tag(map() | Tag.t()) :: Tag.t() | Ecto.Changeset.t()
+  # We need a changeset instead of a raw struct because of slug which is generated in changeset
+  defp process_tag(%{id: id} = _tag) do
+    Events.get_tag(id)
+  end
+
   defp process_tag(tag), do: Tag.changeset(%Tag{}, tag)
 
   defp maybe_put_publish_date(%Changeset{} = changeset) do