formatter.ex: Use Phoenix.HTML for mention/hashtag generation

Unlike concatenating strings, this makes sure everything is escaped.
Tests had to be changed because Phoenix.HTML runs attributes through
Enum.sort before generation for whatever reason.

test/web/twitter_api/twitter_api_test.exs

@@ -109,7 +109,7 @@ defmodule Pleroma.Web.TwitterAPI.TwitterAPITest do
     {:ok, user2} = TwitterAPI.register_user(data2)
 
     expected_text =
-      ~s(<span class="h-card"><a data-user="#{user1.id}" class="u-url mention" href="#{
+      ~s(<span class="h-card"><a class="u-url mention" data-user="#{user1.id}" href="#{
         user1.ap_id
       }" rel="ugc">@<span>john</span></a></span> test)