formatter: use class='mention' on links to user profiles

helps mastodon frontend and apps have better context:
https://mastodon.social/users/Gargron/statuses/100101267352899379

lib/pleroma/formatter.ex

@@ -200,7 +200,7 @@ def add_user_links({subs, text}, mentions) do
           ap_id = info["source_data"]["url"] || ap_id
 
           short_match = String.split(match, "@") |> tl() |> hd()
-          {uuid, "<span><a href='#{ap_id}'>@<span>#{short_match}</span></a></span>"}
+          {uuid, "<span><a class='mention' href='#{ap_id}'>@<span>#{short_match}</span></a></span>"}
         end)
 
     {subs, uuid_text}