Split Federation as separate context

lib/service/metadata/metadata.ex

@@ -0,0 +1,7 @@
+defprotocol Mobilizon.Service.Metadata do
+  @doc """
+  Build tags
+  """
+
+  def build_tags(entity)
+end

lib/service/metadata/utils.ex

@@ -0,0 +1,12 @@
+defmodule Mobilizon.Service.Metadata.Utils do
+  @moduledoc """
+  Tools to convert tags to string.
+  """
+
+  alias Phoenix.HTML
+
+  def stringify_tags(tags), do: Enum.reduce(tags, "", &stringify_tag/2)
+
+  defp stringify_tag(tag, acc) when is_tuple(tag), do: acc <> HTML.safe_to_string(tag)
+  defp stringify_tag(tag, acc) when is_binary(tag), do: acc <> tag
+end