Fix emoji tags for shareable packs to be "pack:{name}"

lib/pleroma/emoji/loader.ex

@@ -99,7 +99,7 @@ defp load_pack(pack_dir, emoji_groups) do
       contents["files"]
       |> Enum.map(fn {name, rel_file} ->
         filename = Path.join("/emoji/#{pack_name}", rel_file)
-        {name, filename, pack_name}
+        {name, filename, ["pack:#{pack_name}"]}
       end)
     else
       # Load from emoji.txt / all files